γ-Protocadherin structural diversity and functional implications

Stochastic cell-surface expression of alpha-, beta-, and gamma-clustered protocadherins (Pcdhs) provides vertebrate neurons with single-cell identities that underlie neuronal self-recognition. Here we report crystal structures of ectodomain fragments comprising cell-cell recognition regions of mouse gamma-Pcdhs gamma A1, gamma A8, gamma B2, and gamma B7 revealing trans-homodimers, and of C-terminal ectodomain fragments from gamma-Pcdhs gamma A4 and gamma B2, which depict cis-interacting regions in monomeric form. Together these structures span the entire gamma-Pcdh ectodomain. The trans-dimer structures reveal determinants of gamma-Pcdh isoform-specific homophilic recognition. We identified and structurally mapped cis-dimerization mutations to the C-terminal ectodomain structures. Biophysical studies showed that Pcdh ectodomains from gamma B-subfamily isoforms formed cis dimers, whereas gamma A isoforms did not, but both gamma A and gamma B isoforms could interact in cis with alpha-Pcdhs. Together, these data show how interaction specificity is distributed over all domains of the gamma-Pcdh trans interface, and suggest that subfamily-or isoform-specific cis-interactions may play a role in the Pcdh-mediated neuronal self-recognition code.