Significance Multiprotein complexes in mammalian cells are thought to form by interactions between domains of mature, fully folded proteins. However, in some cases interprotein interaction is obstructed by “buried” or… Click to show full abstract
Significance Multiprotein complexes in mammalian cells are thought to form by interactions between domains of mature, fully folded proteins. However, in some cases interprotein interaction is obstructed by “buried” or inaccessible binding domains. One such example is the interaction between EBP50 and ezrin, proteins linking the plasma membrane and cytoskeleton; self-association of domains in ezrin masks the site recognized by EBP50. Here, we show EBP50 overcomes this obstacle by cotranslationally binding to nascent ezrin’s otherwise masked domain emerging from the translating ribosome. Our study extends the function of mRNA translation beyond “simple” generation of linear peptide chains that fold into mature proteins for subsequent complex assembly; additionally, cotranslation can facilitate interactions with sterically inaccessible domains to form functional multiprotein complexes. Multiprotein assemblages are the intracellular workhorses of many physiological processes. Assembly of constituents into complexes can be driven by stochastic, domain-dependent, posttranslational events in which mature, folded proteins specifically interact. However, inaccessibility of interacting surfaces in mature proteins (e.g., due to “buried” domains) can obstruct complex formation. Mechanisms by which multiprotein complex constituents overcome topological impediments remain enigmatic. For example, the heterodimeric complex formed by EBP50 and ezrin must address this issue as the EBP50-interacting domain in ezrin is obstructed by a self-interaction that occupies the EBP50 binding site. Here, we show that the EBP50-ezrin complex is formed by a cotranslational mechanism in which the C terminus of mature, fully formed EBP50 binds the emerging, ribosome-bound N-terminal FERM domain of ezrin during EZR mRNA translation. Consistent with this observation, a C-terminal EBP50 peptide mimetic reduces the cotranslational interaction and abrogates EBP50-ezrin complex formation. Phosphorylation of EBP50 at Ser339 and Ser340 abrogates the cotranslational interaction and inhibits complex formation. In summary, we show that the function of eukaryotic mRNA translation extends beyond “simple” generation of a linear peptide chain that folds into a tertiary structure, potentially for subsequent complex assembly; importantly, translation can facilitate interactions with sterically inaccessible domains to form functional multiprotein complexes.
               
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