Ca2+-ATPase of the sarcoplasmic retict membrane glycoprotein associated with (membrane-cytoskeleton interactions/intestinal microvilli/immunocy

On the basis of structural observations, it has been proposed that cytoskeletal organization of the intestinal microvilli could be related to striated muscle structure. We have prepared antibodies against an amphipathic membrane glycoprotein (140 kilodaltons) associated with microvillar cy- toskeleton and investigated its occurrence in striated muscle. Frozen sections of striated muscle were prepared according to the technique of Tokuyasu and visualized by indirect immuno- fluorescence with antibodies to the 140-kilodalton protein. In longitudinal sections the labeling was concentrated mainly in the area of the I band. In cross sections a honeycomb pattern was observed, suggesting that the recognized antigen was probably associated with the periphery of the myofibrils. Ul- trathin frozen sections prepared for electron microscopy re- vealed that this antigen is closely associated with the mem- brane of the sarcoplasmic reticulum. In muscle extracts, the antibodies to the intestinal microvillar 140-kilodalton protein recognized a protein of 100 kilodaltons that comigrates with the Ca2+-ATPase of the sarcoplasmic reticulum. They recog- nized a purified preparation of the Ca2+-ATPase and, more specifically, the trypsin-generated fragment A2, the NH2-ter- minal part of the molecule that is exposed on the cytoplasmic face of the sarcoplasmic reticulum. Although these two pro- teins, expressed in unrelated cells, have a different molecular size and' are inserted in different types of membranes, they share a common structural domain responsible for their cross- reactivity. We propose that this domain could also be responsi- ble for a common function-namely, the bridging of actin fila- ments to membranes. The structural relationship between the organization of the actin fibers in striated muscle and in the cytoskeleton of in- testinal microvilli was recognized several years ago. Both types of microfilaments bind heavy meromyosin molecules in an polarized way (1). Until recently, however, the only molecule known to be shared by both structures was actin. A specific set of actin binding proteins has been characterized in the intestinal microvillar microfilaments (villin (2), 110- kilodalton (kDa) protein (3), fimbrin (4)), as well as calmodu- lin (5). We recently reported that two proteins associated with the cytoskeleton of intestinal microvilli are immunologi- cally related to antigens associated with the periphery of myofibril in striated muscle (6). One of these is the 110-kDa protein, a calmodulin binding protein, which is a major com- ponent bridging laterally the actin filaments to the microvil- lar membrane (3). The calmodulin-l10-kDa complex of chicken has been purified (7). The localization of the 110- kDa protein in intestinal cell has been determined (8, 9) by using antibodies raised against NaDodSO4-denatured anti- gens. The other antigen is a 140-kDa integral membrane gly- coprotein that is associated in vitro with the microvillar cy-