RMP, also known as URI, was described as an unconventional member of the prefoldin family of chaperones that are involved in actin and tubulin folding. Like conventional members of the α-class of PFDs, RMP contains N- and C-terminal α-helical coiled-coil structures connected by two β hairpins. In addition, RMP possesses an RPB5-binding segment and a long C-terminal acidic segment. It is posited that RMP exists as a component of a macromolecular complex within human cells and functions as a molecular scaffold to assemble a PFD complex containing other PFDs and proteins with functions in transcription and ubiquitination. Indeed, evidence is provided that RMP negatively modulates RNA polymerase II-dependent transcription by binding to TFIIF and RBP5 and is involved in mTOR signaling by coordinating the regulation of nutrient availability with gene expression. In accord with its ability to coordinate gene expression with nutrient availability, RMP was shown to be a mitochondrial substrate of S6K1. S6K1-mediated phosphorylation of RMP at Ser371 triggers a series of biochemical events that constitute a negative feedback loop, in part, aimed at restraining S6K1 survival signaling and ensuring that the mitochondrial threshold for apoptosis corresponds to availability of nutrients and growth factors.
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