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Published in final edited form as: Trends Microbiol. 2024 Jan 23;32(8):781–790. doi: 10.1016/j.tim.2024.01.001

HIV-1 Induced Translocation of CPSF6 to Biomolecular Condensates

Katarzyna Bialas 1, Felipe Diaz-Griffero 1,*
PMCID: PMC11263504  NIHMSID: NIHMS1961526  PMID: 38267295

Abstract

Cleavage and polyadenylation specificity factor subunit 6 (CPSF6, also known as CFIm68) is a 68-kDa component of the mammalian cleavage factor I (CFIm) complex that modulates mRNA alternative polyadenylation and determines 3’ untranslated region (UTR) length, an important gene expression control mechanism. CPSF6 directly interacts with the HIV-1 core during infection, suggesting involvement in HIV-1 replication. Here, we review the contributions of CPSF6 to every stage of the HIV-1 replication cycle. Recently, several groups described the ability of HIV-1 infection to induce CPSF6 translocation to nuclear speckles, which are biomolecular condensates. We will discuss the implications for CPSF6 localization in condensates and the potential role of condensate-localized CPSF6 in the ability of HIV-1 to control the cell’s protein expression pattern.

Keywords: CPSF6, HIV-1, biomolecular condensates, nuclear speckles, alternative polyadenylation

Cleavage and polyadenylation specificity factor 6 (CPSF6), also known as CFIm68, is a 68-kDa component of the mammalian cleavage factor I (CFIm) complex, which is required for the maturation of pre-mRNA into functional mRNA. Human CPSF6, located on chromosome 12 (12q15), consists of 6,584 base pairs and produces eight known splice variants. In humans, CPSF6 encodes both a 551-amino acid (aa) protein expressed in various human tissues and a less prevalent 588-aa isoform [1]. According to the Human Protein Atlas, CPSF6 expression is highest in bone marrow and lymph nodes, whereas the pancreas and liver have the lowest expression levels. CPSF6 localizes to the nuclear compartment and consists of the following domains (Figure 1):

Figure 1. Protein domain organization of cleavage and polyadenylation specificity factor subunit 6 (CPSF6).

Figure 1.

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The various domains and functions of human CPSF6. The amino acid (aa) positions for each large domain are shown: RNA-recognition motif in orange (aa 81–157); proline-rich domain in yellow (aa 208–398); and arginine-serine-like domain (RSLD) in red (aa 489–551). Additional interaction motifs are shown in green: CPSF5 interaction motif (aa 116–122), glycine/arginine-rich domain (aa 202–206), the HIV-1 capsid interaction motif (aa 276–290), and the transportin 1 (TNPO1) interaction motif (aa 362–390).

  1. RNA-recognition motif (RRM; aa 81–157) binds CPSF5 [2] and is involdved in nuclear paraspeckle localization [3].

  2. Glycine/arginine-rich motif (aa 202–206) interacts with the nuclear RNA export factor 1 (NXF1), also known as transporter associated with antigen processing (TAP) [2].

  3. Proline-rich domain (aa 208–398) contains both a phenylalanine-glycine (FG) motif (aa 276–290) that binds to the HIV-1 core [1] and a transportin 1 (TNPO1)-interaction motif[4].

  4. Prion-like low complexity region (LCR; aa 261–358) enables binding of CPSF6 to HIV-1 capsid lattices [5].

  5. C-terminal arginine/serine-like domain (RSLD; aa 489–551) binds to TNPO3 [4] and serves as a nuclear localization signal (NLS) [4, 6].

  6. Disordered regions (aa 169–411 and aa 477–551)[7, 8].

The RSLD is sufficient to target CPSF6 to the nucleus [4, 6], and nuclear translocation may involve TNPO3 [9], as the crystal structure of CPSF6 revealed residues within the C-terminal domain able to mediate TNPO3 binding [4].

Analysis of CPSF6 subcellular localization reveals grain-like foci localized to the nuclear compartment [6, 10–12]. Fluorescent microscopy studies revealed that CPSF6 localizes to nuclear paraspeckles and accumulates in structures that partially overlap with nuclear speckles [3]. Analysis of synchronized cells reveals that CPSF6 localization varies throughout the cell cycle [3]. Although the RRM domain of CPSF6 does not contain an NLS, this domain is required for the localization of CPSF6 to paraspeckles [3].

CPSF6 is an evolutionarily conserved protein, with homologs identified in a wide range of eukaryotic organisms, including humans, mice, rats, dogs, chickens, frogs, and yeast [1]. This evolutionary conservation suggests that CPSF6 plays an important role in pre-mRNA processing. According to HomoloGene, the entire CPSF6 protein is conserved across the clade Amniota (Homo sapiens, Pan troglodytes, Canis lupus, Bos taurus, Mus musculus, Rattus norvegicus, Gallus gallus). In addition to numerous vertebrate species, CPSF6 homologs have been reported in arthropods and nematodes.

This review article will summarize the cellular role of CPSF6 and its contribution to cellular homeostasis and cancer. In addition, we will summarize and discuss the contribution of CPSF6 to the different steps of HIV-1 replication with emphasis in nuclear events.

1. The Cellular Role of CPSF6

CPSF6 is a component of the CFIm complex [13], which recognizes and binds specific sequences in the 3’ untranslated region (UTR) of pre-mRNA molecules, playing an essential role in the subsequent cleavage and polyadenylation reactions required to generate mature mRNA molecules [14]. CFIm is a tetramer consisting of two 25-kDa CPSF5 subunits and either two 59-kDa CPSF7 subunits or two CPSF6 subunits (Figure 2A). CFIm regulates the nuclear process known as alternative polyadenylation (APA), which governs the length of the 3’UTR (Figure 2B), ultimately determining the mature mRNA length. APA results in the generation of various mRNA isoforms with significantly different 3’UTR lengths. The 3’UTR contains regulatory elements that determine mRNA metabolic features, including stability, translation, nuclear export, and cellular localization. Interestingly, the CFlm-mediated APA process is modulated at the cellular level, resulting in an altered cellular transcription and translation profiles.

Figure 2. Cleavage and polyadenylation specificity factor subunit 6 (CPSF6) is a component of the mammalian cleavage factor I (CFIm) complex.

Figure 2.

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(A) CPSF6’s role in alternative polyadenylation. CPSF6, a component of the CFIm complex, is involved in the maturation of pre-mRNA into functional mRNA. CPSF6 controls alternative polyadenylation by selecting the polyadenylation signal. Cleavage and polyadenylation are controlled by upstream (UGUA, hexameric A[A/U]UAAA, and U-rich) and downstream (GU-rich and U-rich) cis-elements. UGUA is recognized by the CFIm complex (CPSF52 and either CPSF62 or CPSF72). CPSF5 binds directly to the UGUA element and anchors CPSF6 to the transcript. CPSF6 positions the complex at the appropriate cleavage and polyadenylation site and enhances RNA binding. CFIm complex interacts with the CFIIm complex, which is required for RNA cleavage [82]. The A[A/U]UAAA motif recruits the CPSF complex to promote cleavage stimulation factor (CstF). After cleavage the 3′ end of the transcript is then subjected to poly(A) tail addition by poly(A) polymerase. (B) Alternative polyadenylation. Human genes frequently encode more than one polyadenylation signal. The transcript isoforms that are derived from a single gene with two polyadenylation signals (PASs) are illustrated. When the gene uses the proximal PAS, the mature mRNA contains a short 3’UTR. By contrast, the use of a distal PAS results in an mRNA with a long 3’UTR. The ability of the CFIm complex to use the proximal polyadenylation signal instead of the distal signal is referred to as alternative polyadenylation. GOI, gene of interest.

CPSF6 was first reported in 1996 as a 68-kDa polypeptide purified from CFIm complexes, together with a 59-kDa subunit (CPSF7) and a 25-kDa subunit (CPSF5) [15]. CPSF6 stabilizes the CFIm complex when bound to pre-mRNA molecules, positioning the complex at the appropriate site for cleavage and polyadenylation. The association of CPSF6 with CPSF5 requires the CPSF6 RRM domain. CPSF5 binds directly to the pre-mRNA molecule, anchoring CPSF6 to the transcript [6, 16]. Crystallographic studies have revealed that the CPSF5 dimer associates with two polyadenylation signals (PASs, 5’-UGUAA-3’) and is bound on opposite sides by two CPSF6 RRM domains. Biochemical data suggest that CPSF6 in CFlm enhances RNA binding and facilitates RNA looping [2]. One possibility is that CPSF6 binding to CPSF5 changes the conformation of CPSF5 enhancing its ability to bind RNA.

APA is an important gene expression control mechanism, as approximately 70% of human genes are known to contain alternative polyadenylation signals (PASs) in the 3’UTR (Figure 2B) [14]. CPSF6 depletion induces the transcriptome-wide use of proximal PASs (Figure 2B), shortening the lengths of 3’-UTRs [17–19]. Consistent with these findings, CPSF6 depletion resulted in the arrest of mouse embryo development due to 3’-UTR shortening [20]. Interestingly, Ghosh et al. showed that both CPSF6 depletion and overexpression results in changes of APA patterns [21]. More recent findings have revealed that, in neonatal humans and zebrafish larvae, CPSF6 insufficiency causes a developmental syndrome due to shifts in PAS usage [22]. Taken together, these findings imply that changes in CPSF6 expression affect the activity of the CFIm complex and ultimately affect PAS selection.

Although CSPF6 deficiency leads to various developmental issues [22], CPSF6 upregulation has been reported in various cancer types. However, very little is known regarding the exact mechanisms that regulate CPSF6 expression. Recent studies show that human CPSF6 may be post-transcriptionally repressed by the microRNA miR125b, which physically interacts with an miR125b-binding site within the CPSF6 3’UTR [23]. Overexpression of the transcription factor Snail-1 was also reported to upregulate CPSF6 expression [24]. CPSF6 function may be modulated by Thoc5, a component of the transcription export complex that directly interacts with CPSF6; Thoc5 knockdown decreased the association of CPSF6 with the 5’ region of various genes [25]. Both CSPF6 depletion and overexpression in vivo result in the reduced efficiency of histone pre-mRNA processing, which CPSF6 is involved in through interaction with the N-terminus of the U7 small nuclear ribonucleoprotein–interacting protein Lsm11 [26].

2. CPSF6 and Cancer

CPSF6 has diverse functions, and recent studies show that CPSF6 is upregulated in various cancer types, including breast cancer, leukemia, hepatic carcinoma, esophageal squamous cell carcinoma, gastric cancer, colon cancer, and lung adenocarcinoma [24, 27–37]. Genetic alterations can activate proto-oncogenes, leading to cancer development; however, proto-oncogenes can also be overexpressed without genome alteration. CPSF6 can modulate APA, specifically the selection of early or late PASs in the 3’UTR (Figure 2B), which can alter gene expression and promote oncogenesis [27, 30, 31]. Consistent with this potential role of CPSF6 in oncogenesis, cancer cells have been found to be enriched in mRNAs with shorter 3’UTRs due to APA, leading to potentially oncogenic protein overexpression [38].

In solid tumors, CPSF6 upregulation is associated with enhanced invasiveness [24, 28], which correlates with increased metastatic activity. In aggressive breast cancers, elevated CPSF6 expression is linked to poor patient outcomes [29, 39]. Similarly, CPSF6 upregulation is associated with unfavorable prognosis for patients with esophageal squamous cell carcinoma and gastric cancer [31, 36]. Increased CPSF6 expression is also observed in patients with hepatocellular carcinoma and in hepatocyte-derived Huh-7 carcinoma cells [30, 40]. Moreover, CPSF6 has been identified as a fusion gene partner for tyrosine kinases in multiple malignancies. In the chimeric proteins produced by these fusion genes, CPSF6 association domains facilitate the dimerization and activation of tyrosine kinases. The fusion between CPSF6 and fibroblast growth factor receptor 1 has been observed in hematological malignancies [33], whereas the fusion of CPSF6 with platelet-derived growth factor receptor beta was observed in eosinophilia-associated myeloproliferative neoplasms [32].

3. Role of CPSF6 in HIV-1 Replication

The HIV-1 core, a conical structure composed of approximately 250 hexamers and 12 pentamers formed from monomeric capsid protein subunits, is released into the cellular cytoplasm following the fusion of the HIV-1 membrane with the cellular membrane [41, 42]. The viral core houses 2 copies of the viral RNA genome, which contains all necessary information for viral replication. Following release into the cytoplasm, the viral core is transported into the nuclear compartment, where most of the reverse transcription and uncoating processes occur [11, 43–47]. During the reverse transcription stage, viral RNA is converted into double-stranded viral DNA, which integrates into the cellular genome. New viral particles are produced by the transcription and translation of the viral genome integrated into the cellular DNA.

3.1. Discovery of CPSF6

Lee and colleagues performed a screen to identify novel proteins that naturally block HIV-1 infection, referred to as restriction factors, and found that overexpression of a mouse CPSF6 fragment (aa 1–358) potently restricted HIV-1 infection before the formation of two–long terminal repeat circles [1]. Although the full-length CPSF6 protein localizes to the nucleus, the HIV-1 restricting fragment (CPSF61–358) localizes to the cytoplasm. Viral passage experiments in the presence of CPSF61–358 resulted in viruses bearing an N74D substitution in the capsid protein, which conferred viral resistance against CPSF61–358-mediated restriction, indicating that the capsid protein was the viral determinant for restricted infection [1]. The same investigators also demonstrated that viral inhibition requires the direct interaction between CPSF61–358 and the HIV-1 core [1, 48], but HIV-1 cores containing the N74D mutation in the capsid protein did not interact with CPSF61–358. Subsequently, the Yamashita lab showed that HIV-1 cores containing an A77V mutation in the capsid protein did not interact with CPSF61–358 [49]. Experiments examining the fate of the capsid showed that CPSF61–358 was found to inhibit the HIV-1 uncoating process during infection [9, 50]. Other investigations revealed that CPSF61–358 destabilized in vitro assembled HIV-1 capsid proteins [51, 52].

CPSF6 contains FG repeats (Figure 1), which bind to the HIV-1 core via a pocket that forms between the N-terminal domain of one capsid protein subunit and the C-terminal domain of an adjacent capsid protein subunit within the same hexameric ring. In addition to the FG repeats, CPSF6 also binds to the HIV-1 core through homotypic, prion-like low complexity regions [5]. Interestingly, the CPSF6-HIV-1 core interaction is sensitive to drugs, such as PF74, BI-2, GS-CA1, and lenacapavir [53–55], suggesting that this pocket formation is important for hexamer stability. The binding of PF74 to the viral core triggers uncoating [55, 56], whereas the binding of lenacapavir triggers core stabilization [55, 57]. Importantly, the effect of PF74 on HIV-1 infection depends upon the concentration used [58], generally destabilization of the core during infection can be achieved when using concentrations around ~10 μM.

Despite the inhibitory effects observed for CPSF61–358, overexpression of full-length CPSF6 does not block HIV-1 infection. Depletion of endogenous CPSF6 from human cell lines, primary human T cells, or primary human macrophages has only modest effects on HIV-1 infectivity [10, 55, 59–61].

3.2. HIV-1 induces the translocation of CPSF6 and CPSF5 to nuclear speckles

HIV-1 infection triggers a change in the nuclear localization of CPSF6, which moves from paraspeckles to speckles that colocalize with the nuclear speckle marker SC35 [3, 10, 11] (Figure 3). The change in localization observed for CPSF6 and CPSF5 following HIV-1 infection requires the presence of capsid protein in the nucleus [12, 55]; however, the contribution of this phenomena to HIV-1 replication is not understood. Nuclear speckles are dynamic structures enriched for pre-mRNA and splicing factors [62, 63]. Nuclear speckles are known to be highly transcriptionally active and are capable of regulating gene expression [62]. The finding that HIV-1 infection alters the composition of nuclear speckles implies that HIV-1 infection may be able to alter cellular gene expression by modulating the function of nuclear speckles.

Figure 3. HIV-1 infection induces translocation of cleavage and polyadenylation specificity factor subunit 6 (CPSF6) and CPSF5 to nuclear speckles.

Figure 3.

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The cellular changes observed in CPSF6, CPSF5, and lens epithelium–derived growth factor (LEDGF)/p75 localization following HIV-1 infection are illustrated. In uninfected cells, CPSF6, CPSF5, and LEDGF/p75 are distributed throughout the nucleus (left). Upon HIV-1 infection (right), CPSF6 and CPSF5 are translocated into nuclear speckles, subnuclear structures without membranes that behave like biomolecular condensates. LEDGF/p75 appears to surround nuclear speckles following HIV-1 infection (right).

HIV-1 viruses containing capsid proteins bearing mutations that disrupt interaction with CPSF6, such as N74D or A77V, fail to induce the translocation of CPSF6 and CPSF5 to nuclear speckles, suggesting that the nuclear interaction between the capsid protein and CPSF6 is required for the recruitment of CPSF6 and CPSF5 to nuclear speckles [10–12].

3.3. Role of CPSF6 in the nuclear import of the HIV-1 core

Imaging analysis revealed that wild-type HIV-1 complexes penetrate the nucleus at an average distance of 1.8 μM from the nuclear envelope (NE); however, HIV-1 mutant viruses that lack the ability to bind CPSF6 penetrate the nucleus at an average distance of 0.5 μM from the NE [64, 65]. CPSF6 depletion results in the accumulation of HIV-1 subviral complexes at the nuclear envelope in macrophages, with a modest reduction in infectivity [59]. These results suggest that CPSF6 may directly or indirectly contribute to the nuclear entry of the viral core.

3.4. Role of CPSF6 in HIV-1 reverse transcription and integration

The HIV-1 replication cycle involves reverse transcription of the viral RNA, and integration of the viral genome into the cellular genome. Integration does not occur randomly, tending to favor the integration of the viral genome into interior regions of transcriptionally active genes residing in relatively gene-dense chromatin regions [66]. The selection of HIV-1 genomic integration sites involves CPSF6 and lens epithelium–derived growth factor (LEDGF)/p75 [67, 68]. CPSF6 depletion reduces the proportion of integration events that occur in genes and gene-dense regions without affecting total integration [68, 69]. Under conditions of CPSF6 depletion, HIV-1 displays a reduced preference for integration near activating epigenetic markers, instead favoring gene-sparse regions [68]. These results imply that CPSF6 predominantly prevents HIV-1 integration into heterochromatin. Interestingly, HIV-1 infection in cells with CPSF6 depletion results in integration patterns similar to those observed for infection using a virus incapable of interacting with CPSF6 [68, 70]. In agreement, it has been observed that nuclear speckles containing CPSF6 are locations of ongoing reverse transcription and integration suggesting that nuclear speckles are important for HIV-1 replication [10, 44]. Overall, these results suggest that CPSF6 plays an important role in reverse transcription and integration site selection, albeit a minor effect on infectivity.

3.5. HIV-1 induces the translocation of CPSF5/CPSF6 to nuclear structures that behave as biomolecular condensates

Previous findings suggest that the HIV-1 infection–induced translocation of CPSF6 to nuclear speckles requires the presence of capsid protein in the nuclear compartment [10, 11] (Figure 3). Genetic and pharmacological inhibition of reverse transcription and viral integration had no effects on the translocation of CPSF6 to nuclear speckles, suggesting that neither of these processes is required for translocation [12, 44]. Surprisingly, HIV-1 particles lacking an intact genome also induced the translocation of CPSF6 to nuclear speckles, suggesting that only the viral structural proteins are important for this process [12]. Viral particles containing only the HIV-1 Gag-Pol polyprotein pseudotyped with the vesicular stomatitis virus spike glycoprotein were sufficient to induce the translocation of CPSF6 to nuclear speckles [12]. For example, in human A459 cells, CPSF6 localization to nuclear speckles peaks approximately 24 hours post-infection before declining over the next 4 days, indicating that these structures may be transient. Remarkably, as the percentage of cells containing CPSF6 localized to nuclear speckles declines, the sizes of the remaining nuclear speckles containing CPSF6 increase, which may indicate that these structures undergo fusion during cell division.

Cellular structures containing CPSF6 are affected by osmotic stress and the drug 1,6-hexanediol, which both trigger the rapid disassembly of CPSF6-containing nuclear structures [12], consistent with the dynamic nature of biomolecular condensates [71, 72]. These results indicate that HIV-1 infection triggers the translocation of CPSF6 to biomolecular condensates. Interestingly, HIV-1 infection also triggers the translocation of CPSF5 to biomolecular condensates [12] (Figure 3). CPSF5 forms a complex with CPSF6, which is involved in the regulation of APA. The biomolecular condensate structures may contain the CPSF52–CPSF62 tetramer. However, CPSF7, which forms the CPSF52–CPSF72 tetramer, does not appear to accumulate in biomolecular condensates upon HIV-1 infection. While the biochemical composition of nuclear speckles containing CPSF6/CPSF5 remains unknown, some investigators have observed that prolonged infections lead to an enlargement of these structures [73]. This suggests the intriguing possibility that NS containing CPSF6 may represent non-canonical nuclear speckles.

The mechanism through which viral infection induces the translocation of CPSF6 and CPSF5 to nuclear speckles is not understood. Among many possibilities, the virus may be hijacking the APA machinery (CPSF6 and CPSF5), preventing normal function. Nuclear speckles are biomolecular condensates enriched in proteins involved in pre-mRNA processing to facilitate important functions, including mRNA splicing, transcriptional regulation, and co-transcriptional coupling. A second possibility is that the translocation of CPSF6 and CPSF5 to nuclear speckles affects gene expression.

Investigations examining whether other nuclear proteins are in proximity to or are components of the HIV-1 infection–induced structures containing CPSF6 and CPSF5 revealed that the integration cofactor LEDGF/p75 tri-dimensionally surrounds these structures (Figure 3) [12], suggesting a potential functional link between the biomolecular condensate and LEDGF/p75. Although this functional link requires further exploration, the proximity of LEDGF/p75 to the condensate and the resulting formation of a surrounding layer raises the possibility that a second condensate or environment facilitates infection; however, this possibility remains speculation.

It is important to acknowledge that viruses other than HIV-1 also exploit biomolecular condensates––also known as liquid-liquid phase separation structures––for replication [74]: 1) respiratory syncytial virus utilizes condensates for active replication [75], 2) SARS-CoV-2 induces formation of biomolecular condensates for viral assembly [76], 3) mumps virus is dormant in biomolecular condensates [77], and others.

4. CPSF6 and Other Viruses

In addition to the previously described functions, CPSF6 regulates viral alternative RNA processing [78]. The canine minute virus nuclear protein 1 (NP1) regulates viral capsid protein production by controlling viral pre-mRNA processing. NP1 inhibits polyadenylation and cleavage at the proximal polyadenylation site in the viral genome, allowing for the accumulation of an RNA variant containing the capsid protein–coding sequence. Interestingly, CPSF6 interacts with NP1 in transfected cells and is important for capsid expression, suggesting that CPSF6 and NP1 are both important for the regulation of viral pre-mRNA processing. Similar interactions have been observed between CPSF6 and NP1 from human bocavirus 1 (HBoV1), which causes acute respiratory tract infections. CPSF6 colocalizes with HBoV1 replication centers, and CPSF6 depletion significantly decreases viral capsid protein expression [79].

5. Concluding Remarks.

The puzzling discovery that HIV-1 infection induces the translocation of CPSF6/CPSF5 to nuclear speckles, which exhibited the nature of biomolecular condensates, resulted in more questions than answers. Although the HIV-1 field has obsessed on finding how this protein contribute to the virus replication itself, changes of HIV-1 infection upon CPSF6 depletion are modest. Therefore, the simplest explanation may just be that HIV-1 modulates CPSF6 and CPSF5 function in order to control the transcriptional machinery of the cell; however, this remains an interesting hypothesis. Control of the transcriptional machinery of the cell by HIV-1 is likely to be achieved by modulating the alternative polyadenylation functions of CPSF6 and CPSF5, which results in changes of protein expression. The ability of viruses to modulate the cell’s protein expression patterns in order to facilitate their replication is well-established [80, 81]. Many viruses control protein expression in infected cells using viral proteins to target transcription, mRNA processing and translation. i.e., HSV-1 inhibits splicing by using the viral protein ICP27, Polio virus prevents mRNA export using the viral protein 2A, influenza virus inhibits polyadenylation using the protein NS1, Adenovirus blocks transcription initiation by using the viral protein E1A, Rubella inhibits translation by using the viral capsid protein, Bunya viruses prevent translation by using the viral NS protein, and many others. This implies that viruses have developed multiple mechanisms to control the expression pattern of the cell. In most examples, control of cellular protein expression is achieved in order to provide a permissive environment for viral replication. HIV-1 is likely controlling cellular protein expression through modulation of CPSF6 and CPSF5 functions. Although the potential ability of HIV-1 to control protein expression through CPSF6 and CPSF5 is a very appealing hypothesis, it needs further investigation.

Since only structural HIV-1 proteins are necessary to trigger the translocation of CPSF6 and CPSF5 to nuclear speckles, it’s plausible that the translocation of CPSF6 and CPSF5, which is potentially playing a role in regulating the cell’s protein expression pattern, is induced only when HIV-1 proteins are expressed in human cells actively generating viral proteins. If this holds true, it could have significant implications for identifying HIV-1 reservoirs. An essential challenge in the quest to cure HIV-1 is the persistence of reservoirs in patients undergoing antiretroviral therapy. These reservoirs consist of infected cells that exhibit minimal viral protein expression. However, there’s a potential for these cells to transition into prolific HIV-1 particle producers, leading to a full-blown infection resurgence if antiretroviral treatment is halted. The identification of specific proteins expressed in the surface of cells that compose the reservoir could be used as a tool to identify and eliminate the reservoir. Ultimately, further research is required to comprehensively grasp the significance of CPSF6 and CPSF5 translocation to nuclear speckles in the context of HIV-1 infection and reservoir formation.

Highlights.

Cleavage and polyadenylation specificity factor subunit 6 (CPSF6, also known as CFIm68) is a 68-kDa component of the mammalian cleavage factor I (CFIm) complex.

CPSF6 plays a role in maturation of the 3’ untranslated regions (UTR) of pre-mRNAs modulating mRNA alternative polyadenylation, which determines the length of the 3’ UTR.

HIV-1 induces translocation of CPSF6 and CPSF5 from nuclear paraspeckles to speckles, which are biomolecular condensates.

Since depletion CPSF6 modestly affects the ability of HIV-1 to replicate in primary cell, we hypothesize that the translocation of CPSF6 to speckles is a viral mechanism used to affect alternative polyadenylation, which results in the modulation of cellular expression in favor of the virus, and may have important consequences in the identification of latent viral reservoirs.

HIV-1-induced translocation of CPSF6 to biomolecular condensates may induce a change in the biochemical properties of the protein.

Outstanding Questions Box.

Does translocation of CPSF6 to biomolecular condensates affects alternative polyadenylation of host genes?

What potential benefits, if any, does the virus gain from regulating cellular transcription during infection?

Are any post-translational modifications responsible for HIV-1 induced translocation of CPSF6 from paraspeckles to speckles and if so, are these modifications present in CPSF6 in general or are they specific to HIV-1 infection?

Does CPSF6 change its conformation as a result of HIV-1 induced translocation of CPSF6 from paraspeckles to speckles?

Could any other difference account for CPSF6 localization to speckles versus para speckles?

Footnotes

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