The Hsp90 Cochaperone TTT Promotes Cotranslational Maturation of PIKK Kinases Prior to Complex Assembly

Abstract

Members of the phosphatidylinositol 3-kinase-related kinase (PIKK) family control fundamental processes, including cell growth, DNA damage repair, and transcription. Although their activities are well characterized, little is known about how PIKKs mature and assemble into active complexes. Previous work identified a novel Hsp90 cochaperone, the trimeric TTT complex, which specifically stabilizes PIKKs. Here we provide new insights into the mechanisms by which TTT promotes PIKK maturation de novo. We discovered that, in fission yeast, TTT binds to PIKKs during translation. Structure-function analyses revealed that TTT recognizes newly synthesized PIKKs through their most conserved domain. Although PIKKs function as part of larger complexes, we demonstrate that PIKKs do not engage in cotranslational assembly with their partners. Rather, we accumulated evidence that TTT protects nascent PIKK polypeptides from misfolding and degradation during translation and that PIKKs acquire their native state only after translation is terminated. We conclude that PIKK maturation and assembly are temporally segregated and involve distinct mechanisms, suggesting that the assembly of large multimeric complexes requires both dedicated chaperones and the cotranslational engagement of their subunits.