A novel muscle protein located inside the terminal cisternae of the sarcoplasmic reticulum

Abstract

An immunofluorescence study of adult rat muscle tissues with a polyclonal antibody against the RGD-directed fibronectin receptor of Friend's erythroleukemia cells (α 5β 1-integrin) unexpectedly revealed a pattern of intracellular antigen distribution. Western blotting analysis of rat and rabbit membrane fractions indicated that the antibody recognizes a 167-kDa protein expressed both in heart and in skeletal muscle (relative abundance: heart > slow muscle > fast muscle), but not in liver and kidney. The 167-kDa protein did not show altered electrophoretic mobility upon reduction and failed to bind several lectins, including wheat germ agglutinin. A study of its subcellular distribution in rabbit skeletal muscle revealed that the 167- kDa protein is mostly associated with the terminal cisternae of the sarcoplasmic reticulum (SR) and, to a smaller extent, with the sarcolemma, while it is absent in the longitudinal tubules of the SR. The 167-kDa protein is not an integral membrane protein since it can be extracted at pH ≤10. This protein can be proteolytically cleaved only in the presence of detergent, indicating that it resides on the luminal side of the SR. The 167- kDa protein could be resolved from the closely spaced sarcalumenin and histidine-rich protein by column chromatography followed by detergent dialysis and two-dimensional gel electrophoresis. The N terminus and the internal sequences did not match any known sequence in protein and DNA data bases, indicating that the 167-kDa protein is a novel muscle protein selectively localized to the SR. Integrins from rat kidney fibroblasts were not recognized by either (i) a polyclonal antiserum against the purified 167- kDa protein or (ii) the anti-α 5β 1-integrin antiserum after affinity purification onto the 167-kDa protein. These data indicate that the 167-kDa protein is not immunologically cross-reactive with integrins, despite its reaction with a polyclonal anti-integrin antibody.