p32 is a novel mammalian Lgl binding protein that enhances the activity of protein kinase Cζ and regulates cell polarity

Lgl (lethal giant larvae) plays an important role in cell polarity. Atypical protein kinase C (aPKC) binds to and phosphorylates Lgl, and the phosphorylation negatively regulates Lgl activity. In this study, we identify p32 as a novel Lgl binding protein that directly binds to a domain on mammalian Lgl2 (mLgl2), which contains the aPKC phosphorylation site. p32 also binds to PKC xi, and the three proteins form a transient ternary complex. When p32 is bound, PKC xi is stimulated to phosphorylate mLgl2 more efficiently. p32 overexpression in Madin-Darby canine kidney cells cultured in a 3D matrix induces an expansion of the actin-enriched apical membrane domain and disrupts cell polarity. Addition of PKC xi inhibitor blocks apical actin accumulation, which is rescued by p32 overexpression. p32 knockdown by short hairpin RNA also induces cell polarity defects. Collectively, our data indicate that p32 is a novel regulator of cell polarity that forms a complex with mLgl2 and aPKC and enhances aPKC activity.