Figure 1.

Identification and relative abundance of mRNP components. (a) Oligo-dT-cellulose-selected nuclear hnRNP or cytoplasmic mRNP proteins from [³H]-uridine pulse-labeled HeLa cells. (Lanes 1–3) hnRNP proteins recovered from the unbound fraction (lane 1) or from elution with 25% formamide (lane 2) or with 50% formamide (lane 3). (Lanes 4–5) mRNP proteins in the unbound fraction (lane 4) (mainly ribosomal subunits) or eluted with 50% formamide (lane 5). Panel adapted with permission from plate I in Reference 3. (b) Proteins from oligo-dT-cellulose-selected polyribosomal RNPs from human KB cells labeled with [³⁵S]-methionine without (lane 1) or with (lane 2) short-wave UV irradiation. Oligo-dT-selected proteins are as in lane 2, except the radioactive signal was transferred from [³H]-adenosine- and [³H]-uridine-labeled RNAs after UV cross-linking. Panel modified with permission from Reference 10. (c) [³⁵S]-labeled HeLa hnRNP particle proteins coimmunopurified with 4F4 monoclonal antibody (anti–hnRNP C) and resolved by two-dimensional electrophoresis. Panel modified under a Creative Commons license from Reference 14. (d) Copy numbers of mRNA interactome proteins in HeLa cells. HeLa cell copy number data are from Reference 21, and mRNA interactome data are from Reference 49. The interactome proteins are grouped into categories as shown below the x axis. Some proteins that are absent from the HeLa interactome (red dots) are included for better representation within a category. Shown under each protein category are the estimated copy numbers of its members within an mRNP assembled on an average human mRNA with eight exons and seven introns. EJC copy number is based on ~80% EJC occupancy on human mRNAs (35, 36). The SR protein copy number is based on stoichiometric amounts of SR and SR-like proteins associated with the EJC (36). The number of PABP molecules per mRNA is based on a single PABP occupying ~27 adenosines (144) on an average polyA tail of ×250 nt. Abbreviations: EJC, exon junction complex; hnRNP, heterogeneous nuclear ribonucleoprotein; mRNA, messenger RNA; mRNP, mRNA-containing RNP; PABP, polyadenosine (polyA)-binding protein.