Smooth muscle actomyosin promotes Ca2+-dependent interactions between annexin VI and detergent-insoluble glycosphingolipid-enriched membrane domains

The mechanical link coupling cytoskeletal and contractile proteins to the s arcolemma of smooth muscle cells is essential for transmitting tension from the cell's interior to exterior. In addition to the well-characterized act in-integrin associations present in adhaerens junctions, our recent work ha s postulated the existence of a reversible annexin-dependent membrane-cytos keleton complex, forged in response to a rise in intracellular Ca2+ concent ration following smooth muscle cell stimulation (Babiychuk et al., J. Biol Chem. 1999, 274, 35191-35195). Detailed biochemical characterization of the interactions responsible for the formation of this complex revealed that a nnexins II and VI interact with actomyosin, or detergent-insoluble glycosph ingolipid-enriched membrane domains (rafts) purified from smooth muscle, in a concentration- and Ca2+-dependent manner. Annexin If interacted with lip id rafts with high Ca2+-sensitivity, while for annexin VI this interaction required non-physiologically high concentrations of free Ca2+. However, the Ca2+-sensitivity of the latter interaction strongly increased in the prese nce of purified smooth muscle actomyosin. The detailed biochemical analysis of the interactions occurring between annexin II, annexin VI, actomyosin a nd rafts suggests that annexins regulate sarcolemmal organization during sm ooth muscle cell contraction.