Abstract
How cells specify morphologically distinct plasma membrane domains is poorly understood. Prior work has shown that restriction of microvilli to the apical aspect of epithelial cells requires the localized activation of the membrane-F-actin linking protein ezrin. Using an in vitro system, we now define a multi-step process whereby the kinase LOK specifically phosphorylates ezrin to activate it. Binding of PIP2 to ezrin induces a conformational change permitting the insertion of the LOK C-terminal domain to wedge apart the membrane and F-actin-binding domains of ezrin. The N-terminal LOK kinase domain can then access a site 40 residues distal from the consensus sequence that collectively direct phosphorylation of the appropriate threonine residue. We suggest that this elaborate mechanism ensures that ezrin is only phosphorylated at the plasma membrane, and with high specificity by the apically localized kinase LOK.
Highlights
All nucleated cells can polarize to generate morphologically and biochemically distinct regions at the cell surface
Our results indicate that PIP2 binding to the ezrin FERM domain transmits a conformational change through the a-helical region to weaken the FERM/ezrin-CTD association
We explored the possible role of phospholipid phosphatidylinositol 4,5-bisphosphate (PIP2) as previous reports have suggested a regulatory role for this phospholipid (Fievet et al, 2004; Hao et al, 2009)
Summary
All nucleated cells can polarize to generate morphologically and biochemically distinct regions at the cell surface. The apical and basolateral domains of epithelial cells have distinct protein and lipid compositions, and microvilli are restricted to the apical domain. How cells maintain morphologically distinct regions of their cell surface is not clear. We have been addressing this issue by examining how microvilli are assembled and localized to the apical surface of epithelial cells (Sauvanet et al, 2015). Loss of the single ERM protein is lethal, but when selectively knocked out in photoreceptor cells, microvilli are lost (Karagiosis and Ready, 2004; Saotome et al, 2004; Speck et al, 2003). ERM proteins provide a critical function in polarized morphogenesis
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