A 33,000-dalton protein of the microvilli isolated from rabbit small-intestinal epithelial cells. Purification and some properties.

Actin and four major associated proteins have been identified in the microvillus cytoskeleton from chicken intestinal brush borders isolated in the presence of Ca2+-chelating agents. We have isolated microvilli from rabbit small intestine in the presence of Ca2+ and found by sodium dodecyl sulfate-gel electrophoresis that a protein of 33,000 daltons exists in a molar ratio to actin of 1:3 within the isolated microvillus. The protein could be extracted from acetone powder of microvilli in the absence, but not in the presence, of Ca2+. Thus, for its purification the acetone powder was extracted twice with buffer containing Ca2+ and then with buffer containing a Ca2+-chelator. The 33,000-dalton protein was purified from the last extract by Sephadex G-100 gel filtration and DEAE-cellulose column chromatography. The purified protein was a water-soluble monomeric protein with a molecular weight of 33,000. Antisera against the purified protein raised in guinea pigs reacted with the antigen but not with actin or tropomyosin purified from rabbit skeletal muscle. Freezing-thawing resulted in aggregation of the purified protein, and the aggregates showed filamentous and sheet-like structures. These results suggests that the 33,000-dalton protein is a major component of the microvillus cytoskeleton.