Eb. Babiychuk et al., Annexin VI participates in the formation of a reversible, membrane-cytoskeleton complex in smooth muscle cells, J BIOL CHEM, 274(49), 1999, pp. 35191-35195
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.