Interaction between PAK and Nck: a template for Nck targets and role of PAK autophosphorylation

The kinase PAK binds tightly to the SH3 domain of its partner PIX via a cen tral proline-rich sequence. A different N-terminal sequence allows alpha PA K to bind an SH3 domain of the adaptor Nck. The Nck SH3[2] domain interacts equally with an 18-mer PAK-derived peptide and full-length alpha PAK. Deta iled analysis of this binding by saturation substitution allows related Nck targets to be accurately identified from sequence characteristics alone. A ll Nck SH3[2] binding proteins, including PAK, NIK, synaptojanin, PRK2, and WIP, possess the motif PXXPXRXXS; in the case of PAK, serine phosphorylati on at this site negatively regulates binding. We show that kinase autophosp horylation blocks binding by both Nck and PIX to alpha PAK, thus providing a mechanism to regulate PAK interactions with its SH3-containing partners. One cellular consequence of the regulatable binding of PAK is facilitation of its cycling between cytosolic and focal complex sites.