SECONDARY STRUCTURE AND THERMAL-STABILITY OF CALDESMON AND ITS DOMAINS

Muscle caldesmon is a long, thin protein molecule whose N- and C-terminal regions are separated by a central region which is not present in nonmuscle caldesmon. The three regions appear to be independent structural domains since the α-helical content of intact muscle and liver caldesmon is a sum of the α-helical contents of the component thrombic fragments over a broad temperature range. Based on circular dichroism spectra of liver and muscle caldesmon and its fragments, together with secondary structure prediction algorithms, it is estimated that the N-domain consists of a string of four to five short-to-intermediate-length α-helices; the central domain contains a long continuous α-helical stretch; and the C-domain can be divided into two subregions, the N-terminal C1-region, containing a long α-helix, and the C-terminal C2-region, containing only random coil. The thermal unfolding of caldesmon takes place gradually without a steep transition and the unfolding is reversible upon cooling, consistent with the known 'heat resistance' of caldesmon. This 'continuum-of-states' unfolding contrasts with the sharp, cooperative, two-state unfolding characteristic of many proteins. The domains of caldesmon also unfold gradually with the degree of unfolding increasing in the order C-domain < intact molecule < central domain < N-domain, suggesting that the thermal stability decreases in this order. © 1993 Academic Press, Inc.