Villin, a Ca2(+)-regulated F-actin bundling, severing, capping, and nucleating protein, is a major component of the core of microvilli of the intestinal brush border. Its actin binding properties, tissue specificity, and expression during cell differentiation suggest that it might be involved in the organization of the microfilaments in intestinal epithelial cells to form a brush border. Recently, Friederich et al., (Friederich, E., C. Huet, M. Arpin, and D. Louvard. 1989. Cell. 59:461-475) showed that villin expression in transiently transfected fibroblasts resulted in the loss of stress fibers and the appearance of large cell surface microvilli on some cells. Here, we describe the effect of villin microinjection into cells that normally lack this protein, which has allowed us to examine the immediate and long-term effects of introducing different concentrations of villin on microfilament organization and function. Microinjected cells rapidly lost their stress fibers and the actin was reorganized into abundant villin containing cortical structures, including microspikes and, in about half the cells, large surface microvilli. This change in actin organization persisted in cells for at least 24 h, during which time they had gone through two or three cell divisions. Microinjection of villin core, that lacks the bundling activity of villin but retains all the Ca2(+)-dependent properties, disrupted the stress fiber system and had no effect on cell surface morphology. Thus, the Ca2(+)-dependent activities of villin are responsible for stress fiber disruption, and the generation of cell surface structures is a consequence of its bundling activity. Microinjection of villin led to the reorganization of myosin, tropomyosin, and alpha-actinin, proteins normally associated with stress fibers, whereas both fimbrin and ezrin, which are also components of microvillar core filaments, were readily recruited into the induced surface structures. Vinculin was also redistributed from its normal location in focal adhesions. Despite these changes in the actin cytoskeleton, cells were able to divide and undergo cytokinesis, move, spread on a substratum, and ruffle. Thus, we show that a single microfilament-associated protein can reorganize the entire microfilament structure of a cell, without interfering with general microfilament-based functions like cytokinesis, cell locomotion, and membrane ruffling.

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