Ca2+-ATPase of the sarcoplasmic reticulum shares a common domain with a membrane glycoprotein associated with the cytoskeleton of microvilli.

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 cytoskeleton and investigated its occurrence in striated muscle. Frozen sections of striated muscle were prepared according to the technique of Tokuyasu and visualized by indirect immunofluorescence 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. Ultrathin frozen sections prepared for electron microscopy revealed that this antigen is closely associated with the membrane 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 recognized a purified preparation of the Ca2+-ATPase and, more specifically, the trypsin-generated fragment A2, the NH2-terminal part of the molecule that is exposed on the cytoplasmic face of the sarcoplasmic reticulum. Although these two proteins, 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 crossreactivity. We propose that this domain could also be responsible for a common function--namely, the bridging of actin filaments to membranes.