A Long Helix from the Central Region of Smooth Muscle Caldesmon

Abstract

The central region of smooth muscle caldesmon is predicted to form α-helices on the basis of its primary structure. We have isolated a fragment (CT54) that contains this region. The hydrodynamic properties and the electron microscopic images suggest that CT54 is an elongated (35 nm), monomeric molecule. The circular dichroic spectrum yields an overall α-helical content of 55–58%. These results are consistent with the model that the middle portion of CT54 forms a long stretch of single-stranded α-helix. Such a structure, if it in fact exists, is thought to be stabilized by numerous salt bridges between charged residues at positions i and i+4. The structural characteristics of this fragment not only represent an unusual protein configuration but also provide information about the functional role of caldesmon in smooth muscle contraction. Originally published Journal of Biological Chemistry, Vol. 266, No. 21, July 1991