Figure 4.

Examples of functional overlap of CBs and nucleoli. (A) RNP biogenesis. CBs have long been implicated in the assembly of both the small nuclear ribonucleoproteins (snRNPs) required for pre-mRNA splicing and the small nucleolar ribonucleoproteins (snoRNPs) required for rRNA processing within the nucleolus. snRNP factors (RNA and protein) have also been shown to shuttle through nucleoli during the maturation process, and to accumulate there in response to certain perturbations. (B) Telomerase assembly and trafficking. In early S-phase, human telomerase reverse transcriptase (hTERT) has ben detected in the granular component (GC) and dense fibrillar component (DFC) of the nucleolus (No), while human telomerase RNA (hTR) is found in nucleolus-associated Cajal bodies (CBs). Dyskerin has been detected in the DFC and CBs. Throughout S-phase, when telomerase is active, CBs containing telomerase (hTERT and hTR) have been seen co-localized with telomeres. The multi-functional protein WRAP53/TCAB1 is required for telomerase retention in CBs.