Calcium control of microfilaments: uncoupling of the F-actin-severing and -bundling activity of villin by limited proteolysis in vitro.

Villin is a major F-actin-bundling protein present in the microfilament bundle underlying the plasma membrane of the microvilli present on intestinal epithelial cells. Mild in vitro proteolysis converts villin (Mr, 95,000) into a large fragment, the villin core (apparent Mr, 90,000). Villin core has lost the F-actin-bundling activity expressed by villin in the absence of calcium but retains the micromolar Kd calcium-binding site and the calcium-dependent restriction of actin filament length (F-actin severing) of intact villin. This finding suggests a common structural and functional relatedness between the known calcium-dependent F-actin-severing proteins from different cell types, even though not all of them reveal F-actin-bundling activity.