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 arrange ment 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. Si nce cellular mechanics are undoubtedly regulated by components of the membr ane and cytoskeleton, we have investigated the potential role played by ann exins (a family of phospholipid- and actin-binding, Ca2+-regulated proteins ) in regulating sarcolemmal organization, Stimulation of smooth muscle cell s elicited a relocation of annexin VI from the cytoplasm to the plasmalemma , In smooth, but not in striated muscle extracts, annexins II and VI coprec ipitated with actomyosin and the caveolar fraction of the sarcolemma at ele vated Ca2+ concentrations. Recombination of actomyosin, annexins, and caveo lar lipids in the presence of Ca2+ led to formation of a structured precipi tate. Participation of all 3 components was required, indicating that a Ca2 +-dependent, cytoskeleton-membrane complex had been generated. This associa tion, which occurred at physiological Ca2+ concentrations, corroborates our biochemical fractionation and immunohistochemical findings and suggests th at annexins play a role in regulating sarcolemmal organization during smoot h muscle contraction.