Mapping of caldesmon: relationship between the high and low molecular weight forms

Caldesmon is a widely distributed contractile protein that occurs in both a high molecular weight [120-150-kilodalton (kDa)] and a low molecular weight (71-80-kDa) form, depending on the tissue. The structural relationship between these two forms was examined by mapping techniques. Partial cyanogen bromide cleavage in conjunction with sodium dodecyl sulfate gel electrophoresis was used to construct a map of the cleavage points and determine the relative position of the fragments in a high molecular weight caldesmon from chicken gizzard (caldesmon125). By use of this map, markers for different regions of the protein were obtained: Antibodies directed toward certain areas were prepared by affinity purification, and specific 125I-labeled tryptic peptides were found to originate from terminal cyanogen bromide fragments. Mapping of a lower molecular weight form of caldesmon (caldesmon72 from chicken liver) revealed the presence of sequences located in both ends of caldesmon125. A terminal 38-kDa fragment of both proteins was apparently identical on the basis of arrangement of cleavage sites, antibody reactivity, and iodopeptide mapping. Fragments from the other end of both proteins exhibited an identical pattern of peptides. These results show that it is sequences located in the central area of caldesmon125 which are missing in caldesmon72, indicating that the smaller molecule is not simply a proteolytic product of the larger. The two forms of caldesmon may be derived from separate genes or by alternative splicing from a single gene.