Spatiotemporal phosphoregulation of Lgl: Finding meaning in multiple on/off buttons

Abstract

Intracellular asymmetries, often termed cell polarity, determine how cells organize and divide to ultimately control cell fate and shape animal tissues. The tumor suppressor Lethal giant larvae (Lgl) functions at the core of the evolutionarily conserved cell polarity machinery that controls apico-basal polarization. This function relies on its restricted basolateral localization via phosphorylation by aPKC. Here, we summarize the spatial and temporal control of Lgl during the cell cycle, highlighting two ideas that emerged from our recent findings: 1) Aurora A directly phosphorylates Lgl during symmetric division to couple reorganization of epithelial polarity with the cell cycle; 2) Phosphorylation of Lgl within three conserved serines controls its localization and function in a site-specific manner. Considering the importance of phosphorylation to regulate the concentration of Lgl at the plasma membrane, we will further discuss how it may work as an on-off switch for the interaction with cortical binding partners, with implications on epithelial polarization and spindle orientation.

Keywords: Dlg; Drosophila; Lgl; aPKC; basolateral; cell polarity; cell-cycle; epithelial tissue; phosphorylation; spindle orientation.

FIGURE 1.

AurA and aPKC control Lgl localization during the cell cycle in epithelia. (A) A conserved Basic and Hydrophobic (BH) domain of Lgl contains the three Serines controlled by aPKC and AurA. Positively charged residues are in blue. (B) Phosphorylation by aPKC on the apical domain controls basolateral restriction of Lgl during interphase. Multisite phosphorylation blocks membrane and cortical interactions, whereas 1P-Lgl may still interact with the plasma membrane, and may be required for interaction with some cortical binding partners. At mitosis onset, activation of Aurora A leads to multisite phosphorylation of Lgl inducing dissociation from the basolateral cortex.

FIGURE 2.

Model of phosphorylation as a molecular on/off switch of Lgl interactions. The establishment of electrostatic interactions with phospholipids concentrates Lgl at plasma membrane (PM). Sequential phosphorylation progressively reduces the electrostatic interactions established by Lgl with cortical Myosin and phosphoinositides. In contrast, one phosphorylation seems to be required for the association of Lgl with the GUK domain of Dlg. Phosphorylation may therefore displace Lgl that was bound to the plasma membrane, allowing binding to specific cortical binding partners. Multisite phosphorylation blocks Dlg-Lgl interaction, possibly by further reducing the local concentration of Lgl or by inducing conformational/electrostatic changes that directly block Dlg-Lgl binding.

FIGURE 3.

Site-specific control of Lgl function and localization. Schematic overview of the localization of singly phosphorylatable and non-phosphorylatable Lgl mutant forms during epithelial interphase and S2 cell mitosis. Based on the ability of each mutant form to support epithelial polarity or to induce dominant basolateral activity upon strong overexpression, we speculate about the activity of each phosphorylatable site (left).