Defining the Caprin‑1 Interactome in Unstressed and Stressed Conditions

Cytoplasmic stress
granules (SGs) are dynamic foci containing translationally
arrested mRNA and RNA-binding proteins (RBPs) that form in response
to a variety of cellular stressors. It has been debated that SGs may
evolve into cytoplasmic inclusions observed in many neurodegenerative
diseases. Recent studies have examined the SG proteome by interrogating
the interactome of G3BP1. However, it is widely accepted that multiple
baits are required to capture the full SG proteome. To gain further
insight into the SG proteome, we employed immunoprecipitation coupled
with mass spectrometry of endogenous Caprin-1, an RBP implicated
in mRNP granules. Overall, we identified 1543 proteins that interact
with Caprin-1. Interactors under stressed conditions were primarily
annotated to the ribosome, spliceosome, and RNA transport pathways.
We validated four Caprin-1 interactors that localized to arsenite-induced
SGs: ANKHD1, TALIN-1, GEMIN5, and SNRNP200. We also validated these
stress-induced interactions in SH-SY5Y cells and further determined
that SNRNP200 also associated with osmotic- and thermal-induced SGs.
Finally, we identified SNRNP200 in cytoplasmic aggregates in amyotrophic
lateral sclerosis (ALS) spinal cord and motor cortex. Collectively,
our findings provide the first description of the Caprin-1 protein
interactome, identify novel cytoplasmic SG components, and reveal
a SG protein in cytoplasmic aggregates in ALS patient neurons. Proteomic
data collected in this study are available via ProteomeXchange
with identifier PXD023271.