Assembly of Proteins by Free RNA during the Early Phase of Proteostasis Stress

At any stage of their lifecycle,
mRNAs are coated by specialized
proteins. One of few circumstances when free mRNA appears in the cytosol
is the disassembly of polysomes during the stress-induced shutdown
of protein synthesis. Using quantitative mass spectrometry, we sought
to identify the free RNA-interacting cellular machinery in heat-shocked
mammalian cells. Free RNA-associated proteins displayed higher disorder
and larger size, which supports the role of multivalent interactions
during the initial phase of the association with RNAs during stress.
Structural features of the free RNA interactors defined them as a
subset of RNA-binding proteins. The interaction between these assembled
proteins in vivo required RNA. Reconstitution of
the association process in vitro indicated a multimolecular
basis for increased binding to RNA upon heat shock in the cytosol.
Our study represents a step toward understanding how free RNA is processed
in the cytosol during proteostasis stress.