Annexin VI participates in the formation of a reversible, membrane- cytoskeleton complex in smooth muscle cells

The plasmalemma of smooth muscle cells is periodically banded. This arrangement ensures efficient transmission of contractile activity, via the firm, actin-anchoring regions, while the more elastic caveolae-containing 'hinge' regions facilitate rapid cellular adaptation to changes in cell length. Since cellular mechanics are undoubtedly regulated by components of the membrane and cytoskeleton, we have investigated the potential role played by annexins (a family of phospholipid- and actin-binding, Ca²⁺-regulated proteins) in regulating sarcolemmal organization. Stimulation of smooth muscle cells elicited a relocation of annexin VI from the cytoplasm to the plasmalemma. In smooth, but not in striated muscle extracts, annexins II and VI coprecipitated with actomyosin and the caveolar fraction of the sarcolemma at elevated Ca²⁺ concentrations. Recombination of actomyosin, annexins, and caveolar lipids in the presence of Ca²⁺ led to formation of a structured precipitate. Participation of all 3 components was required, indicating that a Ca²⁺-dependent, cytoskeleton-membrane complex had been generated. This association, which occurred at physiological Ca²⁺ concentrations, corroborates our biochemical fractionation and immunohistochemical findings and suggests that annexins play a role in regulating sarcolemmal organization during smooth muscle contraction.