Kedersha et al. describe how phosphorylation and the competition between mutually exclusive binding partners regulate G3BP’s ability to mediate stress granule assembly.
When translation initiation is inhibited, mammalian polysomes disassemble into ribonucleoprotein particles that form microscopically visible stress granules (SGs). The RNA-binding protein G3BP can nucleate SG assembly—its overexpression induces SG formation even in translation competent cells—but how the protein does this, and how its activity is regulated, remains unknown.
Kedersha et al. found that cells lacking both isoforms of G3BP were unable to form SGs in response to stresses that inhibit translation initiation via the phosphorylation of eIF2α, though they were still capable of assembling SGs in response to other insults, such as osmotic stress. G3BP can be phosphorylated on a serine residue in a disordered region adjacent to its N-terminal domain. Unlike a nonphosphorylatable version of G3BP1, a phosphomimetic mutant failed to rescue SG assembly in G3BP-deficient cells, indicating that the protein’s activity is regulated by phosphorylation.
Two proteins—Caprin1 and USP10—bind to a region of G3BP neighboring this critical phosphorylation site. Kedersha et al. found that the proteins compete with each other for G3BP binding and that, whereas Caprin1 promotes SG formation, USP10’s interaction with G3BP inhibits SG assembly. G3BP phosphorylation may regulate this competition, because the phosphomimetic version of G3BP preferentially bound to USP10.
G3BP is a highly dynamic protein that shuttles in and out of SGs. Lead author Nancy Kedersha thinks that phosphorylation and Caprin1/USP10 binding may regulate this shuttling process by altering the protein’s conformation.