Editors' ChoiceBiochemistry

How the Par complex excludes proteins

Sci. Signal.  03 Nov 2015:
Vol. 8, Issue 401, pp. ec320
DOI: 10.1126/scisignal.aad7649

Atypical protein kinase C (aPKC) and the scaffold proteins Par3 and Par6 form the Par complex, which controls cell polarity in various cell types by excluding specific proteins from regions of the cell cortex where the Par complex is present. Par3 and Par6 bind to various proteins, which are then phosphorylated by aPKC and excluded from Par-positive areas of the cellular periphery. Bailey and Prehoda identified a phosphorylation-regulated basic and hydrophic (PRBH) motif that was predicted to mediate protein interaction with the membrane. The PRBH motif was present in Lgl, Numb, and Mira, which are three Par substrates in Drosophila melanogaster that lack sequence or structural similarity to one another. Each PRBH motif overlapped a known or predicted aPKC phosphorylation site. In cultured Drosophila S2 cells, expression of forms of the proteins lacking the PRBH motif remained cytosolic and did not accumulate at the cell cortex, and enhanced green fluorescent protein (EGFP) fused to any of these three PRBH motifs accumulated at the cell cortex. Overexpression of aPKC reduced cortical enrichment of each of the three EGFP-PRBH fusion proteins but not when the aPKC phosphorylation sites in the PRBH motifs were mutated. In vitro, tagged peptides representing each of the three PRBH motifs bound to vesicles containing negatively charged phospholipids, and the presence of aPKC prevented the PRBH motif peptides from interacting with vesicles unless the aPKC phosphorylation sites were mutated. The authors identified similar PRBH motifs in proteins from the sponge, nematode, fruit fly, and human proteomes. The PRBH motifs of several of these candidates were phosphorylated by aPKC in vitro and mediated cortical localization that was prevented by aPKC overexpression in S2 cells. All PRBH-containing proteins may not be substrates for Par-mediated exclusion, because aPKC-mediated phosphorylation also controls membrane association in other contexts independently of the Par complex. Because many kinases have similar specificity, some of these PRBH motifs may be targeted by kinases other than aPKC. Indeed, the authors identified PRBHs in two human proteins with membrane association that is regulated by conventional PKC. Thus, PRBH motifs may be commonly employed to mediate cortical localization that is regulated by phosphorylation.

M. J. Bailey, K. E. Prehoda, Establishment of Par-polarized cortical domains via phosphoregulated membrane motifs. Dev. Cell 35, 199–210 (2015). [PubMed]

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