Nonmuscle myosin IIA is required for lamellipodia formation through binding to WAVE2 and phosphatidylinositol 3,4,5-triphosphate

► Human nonmuscle myosin IIA heavy chain MYH9 is a phosphatidylinositol 3,4,5-triphosphate (PIP 3)-binding protein in breast cancer cells. ► MYH9 constitutively binds to the actin cytoskeletal regulatory protein WAVE2 and the MYH9-WAVE2 complex is translocated to PIP 3 at the leading edge of migrating cells in response to insulin-like growth factor I. ► MYH9 may be necessary for lamellipodia formation and cell migration by providing contractile forces and tension for the actin filament network to form convex arcs. Investigation of the mechanism underlying cell membrane-targeted WAVE2 capture by phosphatidylinositol 3,4,5-triphosphate (PIP 3) through IRSp53 revealed an unidentified 250-kDa protein (p250) bound to PIP 3. We identified p250 as nonmuscle myosin IIA heavy chain (MYH9) by mass spectrometry and immunoblot analysis using anti-MYH9 antibody. After stimulation with insulin-like growth factor I (IGF-I), MYH9 colocalized with PIP 3 in lamellipodia at the leading edge of cells. Depletion of MYH9 expression by small interfering RNA (siRNA) and inhibition of myosin II activity by blebbistatin abrogated the formation of actin filament (F-actin) arcs and lamellipodia induced by IGF-I. MYH9 was constitutively associated with WAVE2, which was dependent on myosin II activity, and the MYH9-WAVE2 complex colocalized to PIP 3 at the leading edge after IGF-I stimulation. These results indicate that MYH9 is required for lamellipodia formation since it provides contractile forces and tension for the F-actin network to form convex arcs at the leading edge through constitutive binding to WAVE2 and colocalization with PIP 3 in response to IGF-I.