Lgl mediates apical domain disassembly by suppressing the PAR-3-aPKC-PAR-6 complex to orient apical membrane polarity

Abstract

The basolateral tumor suppressor protein Lgl is important for the regulation of epithelial cell polarity and tissue morphology. Recent studies have shown a physical and functional interaction of Lgl with another polarity-regulating protein machinery, the apical PAR-3-aPKC-PAR-6 complex, in epithelial cells. However, the mechanism of Lgl-mediated regulation of epithelial cell polarity remains obscure. By an siRNA method, we here show that endogenous Lgl is required for the disassembly of apical membrane domains in depolarizing MDCK cells induced by Ca2+ depletion. Importantly, this Lgl function is mediated by the suppression of the apical PAR-3-aPKC-PAR-6 complex activity. Analysis using 2D- or 3D-cultured cells in collagen gel suggests the importance of this suppressive regulation of Lgl on the collagen-mediated re-establishment of apical membrane domains and lumen formation. These results indicate that basolateral Lgl plays a crucial role in the disassembly of apical membrane domains to induce the orientation of apical membrane polarity, which is mediated by the suppression of apical PAR-3-aPKC-PAR-6 complex activity.

Phosphorylation-dependent interaction between tumor suppressors Dlg and Lgl

“…When unphosphorylated, apical cell cortex-localized Lgl can interact with and inhibit the enzyme activity of aPKC [15,26]. Upon phosphorylation, Lgl undergoes conformational changes causing its dissociation from the apical cortex and concomitant release of its inhibition on aPKC, and subsequent accumulation at the basal-lateral membranes [26,27].…”

“…When unphosphorylated, apical cell cortex-localized Lgl can interact with and inhibit the enzyme activity of aPKC [15,26]. Upon phosphorylation, Lgl undergoes conformational changes causing its dissociation from the apical cortex and concomitant release of its inhibition on aPKC, and subsequent accumulation at the basal-lateral membranes [26,27]. Notably, expression of an unphosphorylatable Lgl (Lgl2 3A) led to severe defects in the segregation of cell fate determinants in the asymmetric cell division of Drosophila neuroblasts [28][29][30], probably due to constitutive inhibition of aPKC at the apical cortex.…”

Regulation of epithelial cell polarity by PAR-3 depends on Girdin transcription and Girdin–Gαi3 signaling

“…The activity of the PAR-3-aPKC-PAR-6 complex is suppressed by Lgl, which competes with PAR-3 for binding to the aPKC-PAR-6 complex (Yamanaka et al, 2006(Yamanaka et al, , 2003. We performed an immunoprecipitation analysis using anti-PAR-3 antibody to evaluate the association between PAR-3 and aPKC, and show that more aPKCλ could be coprecipitated with PAR-3 from MDCK cells depleted of the two mammalian Lgl homologs than from control cells (Fig.…”

“…Studies using this system have uncovered the molecular basis of the opposing actions of PAR-3 and Lgl in epithelial cell polarity regulation (Horikoshi et al, 2009;Yamanaka et al, 2006Yamanaka et al, , 2003. To gain further insight into events downstream of the PARaPKC complex, we searched for genes that are reciprocally regulated by PAR-3 and Lgl.…”

“…In addition, by binding to PAR-3, the LIMK2 and Tiam1 actin cytoskeleton regulatory proteins and the Sec 8 exocyst protein (also known as EXOC4) have been implicated in epithelial cell polarization (Bryant et al, 2010;Macara, 2005, 2006;Zuo et al, 2009). Moreover, the Lgl tumor suppressor protein (for which there are two isoforms in mammals, LLGL1 and LLGL2) negatively regulates the tripartite PAR-3-aPKC-PAR-6 complex by competing with PAR-3 for binding to the aPKC-PAR-6 complex (Chalmers et al, 2005;Hutterer et al, 2004;Yamanaka et al, 2006Yamanaka et al, , 2003. These results show that the PAR-aPKC complex has a central role in establishing epithelial cell polarity through coordinating regulatory proteins of the cytoskeletal and vesicular transport systems.…”