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 alpha-helical content of intact muscle and liver caldesmon is a sum of the alpha-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 alpha-helices; the central domain contains a long continuous alpha-helical stretch; and the C-domain can be divided into two subregions, the N-terminal C1-region, containing a long alpha-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.
Graceffa
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Department of Muscle Research, Boston Biomedical Research Institute, Massachusetts 02114.
Jancsó
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D000818:Animals; D002148:Calmodulin-Binding Proteins / Q000737:chemistry*; D002645:Chickens; D002942:Circular Dichroism; D004355:Drug Stability; D005895:Gizzard, Avian; D006358:Hot Temperature; D008099:Liver; D009130:Muscle, Smooth; D017433:Protein Structure, Secondary; D011817:Rabbits; D013056:Spectrophotometry, Ultraviolet; D013816:Thermodynamics
D002148:Calmodulin-Binding Proteins