A divergent tubulin-like protein templates eukaryotic chaperonin assembly

The CCT chaperonin uses a specific ring topology of eight related subunits to instruct tubulin folding by site-specific interactions. Subunits comprising the negatively-charged hemisphere (Cct2/4/5/7) have the potential to self-assemble but how the positively charged subunits (Cct1/3/6/8) come together to complete the CCT chamber has remained unclear. We used complementary machine learning and biochemical screening approaches to identify a tubulin-like CCT assembly factor conserved from yeast (Dml1) to humans (MSTO1). Our experimental evidence supports a model in which Cct6 binding to Dml1 is followed by cooperative recruitment of Cct3 and Cct1; Dml1 then co-opts a tubulin-like E-hook, used normally in tubulin for cytoskeletal motor binding, to enable half-ring joining by electrostatic bridging coupled to Cct8 incorporation. Molecular mimicry is a recurring theme in ribosome subunit assembly; here we show that it also underlies the unexpected reciprocal cooperation between CCT and tubulin gene families.