Energetic determinants of the Par-3 interaction with the Par complex

The animal cell polarity regulator Par-3 recruits the Par complex (Par-6 and atypical Protein Kinase C–aPKC) to specific sites on the cell membrane. Although numerous physical interactions have been reported between Par-3 and the Par complex, it has been unclear how each contributes to the overall interaction. Using purified, intact Par complex and a quantitative binding assay, we found that energy for this interaction is provided by Par-3’s second and third PDZ protein interaction domains. Both Par-3 PDZ domains bind to aPKC’s PDZ Binding Motif (PBM) in the Par complex, with binding energy contributed from aPKC’s adjacent catalytic domain. In addition to highlighting the role of Par-3 PDZ interactions with the aPKC kinase domain and PBM in stabilizing Par-3 – Par complex assembly, our results indicate that each Par-3 molecule can potentially recruit two Par complexes to the membrane during cell polarization.