Multi-body cryo-em maps and models of a pentameric KCTD5/Cullin3/Gβγ E3 ubiquitin ligase complex
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Heterotrimeric G proteins can be regulated by post-translational modifications, including ubiquitylation. KCTD5, a pentameric substrate receptor protein consisting of an N-terminal BTB domain and a C-terminal domain (CTD), engages CUL3 to form the central scaffold of a cullin-RING E3 ligase complex (CRL3<sup>KCTD5</sup>) that ubiquitylates Gβγ and reduces Gβγ protein levels in cells. The cryo-EM structure of a 5:5:5 KCTD5/CUL3<sup>NTD</sup>/Gβ<sub>1</sub>γ<sub>2</sub> assembly reveals a highly dynamic complex with rotations of over 60° between the KCTD5<sup>BTB</sup>/CUL3<sup>NTD</sup> and KCTD5<sup>CTD</sup>/Gβγ moieties of the structure. CRL3<sup>KCTD5</sup> engages the E3 ligase ARIH1 to ubiquitylate Gβγ in an E3-E3 super-assembly, and extension of the structure to include full-length CUL3<sup> </sup>with RBX1 and an ARIH1~ubiquitin conjugate reveals that some conformational states position the ARIH1~ubiquitin thioester bond to within 10 Å of lysine-23 of Gβ and likely represent priming complexes. Most previously described CRL/substrate structures have consisted of monovalent complexes and have involved flexible peptide substrates. The structure of the KCTD5/CUL3<sup>NTD</sup> Gβγ complex shows that the oligomerization of a substrate receptor can generate a polyvalent E3 ligase complex and that the internal dynamics of the substrate receptor can position a structured target for ubiquitylation in a CRL3 complex.
