Possible involvement of myosin-X in intercellular adhesion: importance of serial pleckstrin homology regions for intracellular localization.

Subcellular fractionation experiments with mouse hepatocytes, combined with sodium dodecylsulfate (SDS)-polyacrylamide gel electrophoresis (PAGE)-immunoblot analysis using antibodies against two different tail regions of mouse myosin-X demonstrated a 240 kDa molecular mass to be associated with the plasma membrane-rich P2 fraction. The basolateral plasma membrane fraction, but not the brush border fraction, isolated from renal cortices also contained the 240 kDa form of myosin-X. In an attempt to assess relative contributions of possible functional domains in the tail of myosin-X to localization and function, cDNA corresponding to all three pleckstrin homology (PH) domains and different regions (PH1, 2 and 3, and the two subdomains of PH1: PHS1 and PHS2), as well as the myosin tail homology 4 domain (MyTH4) and the band4.1/ezrin/radixin/moesin-like domain (FERM) were separately inserted into the pEGFP vector and expressed in cultured COS-1 cells. As a result, two distinct regions responsible for localization were identified with regard to PH: one covers all three forms that tends to localize to regions of dynamic actin, such as membrane ruffles, lamellipodia and thick cortical actin bundles at the sites of cell-cell adhesion in a Rac- and Cdc42-dependent manner. The other covers PHS1 and PH2 that localizes to filopodia, filopodial puncta and the sites of intercellular adhesion in a Cdc42-dependent manner. Expression of green fluorescent protein (GFP)-MyTH4 fusion protein resulted in formation of phalloidin-positive granules, while GFP-FERM affected the actin cytoskeletal system in a distinctly different way. Taken altogether, the results lend support to the view that myosin-X is involved in cell-cell adhesion-associated signaling-linked membrane and/or cytoskeleton reorganization.