Structure of a cell polarity regulator, a complex between aPKC and Par6 PB1 domains

A complex of atypical PKC and Par6 is a common regulator for cell-polarity related processes, which is an essential clue to evolutionary conserved cell-polarity regulation. Here, we determined the crystal structure of the complex of PKCι and Par6α PB1 domains to a resolution of 1.5 Å. Both PB1 domains adopt a ubiquitin fold. PKCι PB1 presents an OPCA motif, 28 amino acid residues with acidic and hydrophobic residues, which interacts with the conserved lysine residue of Par6α PB1 in a front-and-back manner. On the interface, several salt bridges are formed including the conserved acidic residues on the OPCA motif of PKCι PB1 and the conserved lysine residue on the Par6α PB1. Structural comparison of the PKCι and Par6α PB1 complex with the p40 phox and p67 phox PB1 domain complex, subunits of neutrophil NADPH oxidase, reveals that the specific interaction is achieved by tilting the interface so that the insertion or extension in the sequence is engaged in the specificity determinant. The PB1 domain develops the interaction surface on the ubiquitin fold to increase the versatility of molecular interaction.