The Ca(2+)-sensitizing component of smooth muscle thin filaments: properties of regulatory factors that interact with...

We have investigated the nature of the native Ca(2+)-sensitizing factor of smooth muscle native thin filaments. Two different types of preparation yielded protein mixtures which consistently conferred Ca(2+)-sensitivity on thin filaments reconstituted from aorta actin, tropomyosin and caldesmon. These new preparations produced a good model of native thin filaments, unlike previous systems reconstituted with calmodulin or S. 100. We conclude that the Ca(2+)-sensitizing factor is a protein of molecular weight in the range 15 kDa - 40 kDa which is heat-labile and which has properties distinct from calponin, S. 100 or calmodulin.