Domain architecture of the smooth-muscle plasma membrane: regulation by annexins

Individual signalling events are processed in distinct, spatially segregated domains of the plasma membrane. In a smooth muscle, the sarcolemma is divided into domains of focal adhesions alternating with caveolae-rich zones, both harbouring a specific subset of membrane-associated proteins. Recently, we have demonstrated that the sarcolemmal lipids are similarly segregated into domains of cholesterol-rich lipid rafts and glycerophospholipid-rich non-raft regions. In the present study, we provide a detailed structural analysis of the relationship between these proteinaceous and lipid domains. We demonstrate that the segregation of plasmalemmal protein constituents is intimately linked to that of the membrane lipids. Our results imply that lipid segregation is critical for the preservation of membrane protein architecture and essential for directional translocation of proteins to the sarcolemma. We show that the membrane lipid segregation is supported by the annexin protein family in a Ca2+-dependent manner. Eukaryotic cells harbour numerous, tissue-specific subsets of annexins. By examining the significance of this variety in a smooth muscle, we demonstrate that four different annexins target membrane sites of distinct lipid composition and that each annexin requires a different [Ca2+] for its translocation to the sarcolemma. Our results suggest that the interactions of annexins with distinct plasma membrane regions promote membrane segregation and, in combination with their individual Ca2+ sensitivity, might allow a spatially confined, graded response to a multitude of extra- or intracellular stimuli.