Proteomic analysis reveals a role for the GTPase RPAP4/GPN1 and the cochaperone RPAP3 in biogenesis of all three nuclear RNA polymerases

Abstract

Biogenesis of nuclear RNA polymerases (RNAP) is a poorly understood, yet central molecular process in eukaryotes. Recent analysis of interaction partners of RNAP II, the enzyme that synthesizes protein-coding mRNAs, in the soluble fraction of cell extracts identified a series of factors that play central roles in RNAP II biogenesis. The GPN loop GTPase RPAP4/GPN1 was shown to be required for nuclear import of RNAP II, and the HSP90 co-factor RPAP3 is essential for cytoplasmic assembly of this multisubunit enzyme. Examination of the list of interactors for RNAP II as well as RPAP4/GPN1 and RPAP3 reveals the presence of many specific subunits of RNAP I and III, which synthesize most of the cell's non-coding transcripts. This finding suggests that biogenesis of all three nuclear RNAPs may be coupled. Silencing of RPAP4/GPN1 and RPAP3 further indicates that both factors are essential for normal nuclear localization of the three polymerases. We present a model in which biogenesis of RNAP I, II and III is integrated through the action of assembly and nuclear import factors.