Analysis of cytoskeletal proteins and Ca 2+ -dependent regulation of structure in intestinal brush borders from rachitic chicks

Abstract

Several structural aspects of the intestinal epithelial brush border from rachitic chicks were investigated. At the light microscope and EM levels, rachitic brush borders are indistinguishable from controls. Although several of the prominent periodic acid-Schiff-positive proteins of the brush border membrane have slightly slower mobilities on sodium dodecyl sulfate/polyacrylamide gels than do corresponding proteins from control brush borders, the major components of the microvillus core, including subunits of 105, 95 and 68 kilodaltons (KD), actin and calmodulin, are not detectably different. As assayed by a 125I-labeled calmodulin gel overlay technique, the same calmodulin-binding proteins are present in rachitic and control brush borders. Two proteins, the 105-kD subunit of the microvillus core and an .apprx. 30-kD membrane protein, bind calmodulin in a Ca-independent manner. Four cytoskeletal proteins (250, 190, 180 and 150 kd) and 1 membrane protein (35 kD) bind calmodulin only in the presence of Ca. Ca-dependent solation of microvillus core proteins and Ca-dependent phosphorylation of the 20-kD light chain of brush border myosin both occur as in controls. Rachitic chicks apparently have brush borders that are quite similar to controls with respect to their ultrastructural organization, constituent contractile proteins and Ca-dependent regulation of contractility and microvillus core structure. The decreased Ca absorption by intestinal epithelial cells in rachitic chicks is probably not due to gross differences in the brush border cytoskeleton.