Solution X-ray Scattering Data Show Structural Differences among Chimeras of Yeast and Chicken Calmodulin:  Implications for Structure and Function

We present here the first evidence, obtained by the use of small-angle X-ray scattering, of the solution structures of chimeras constructed from yeast (Saccharomyces cerevisiae, Sc) and chicken (Gallus gallus, Gg) calmodulin (CaM). The chimeric proteins used in this study are Sc1-129/Gg130-148, Sc1-128/Gg129-148, Sc1-87/Gg88-148, and Sc1-72/Gg73-148 CaMs, in which Sc1- n and Gg( n +1)-148 descend from yeast and chicken CaM in the chimeric proteins, respectively. Under the Ca2+-saturated condition, the solution structure of Sc1-128/Gg129-148 CaM has a dumbbell-like shape which is characteristic of vertebrate-type CaM, while that of Sc1-129/Gg130-148 CaM takes an intermediate structure between the dumbbell-like shape and a compact globular shape. The results provide the direct evidence that the replacement of Asp129 with Ser129 induces an interaction between two lobes of Sc1-129/Gg130-148 CaM and brings them close together. It implies that a site interacting with the N-lobe is induced in the C-lobe, although site IV that is unable to bind Ca2+ hinders the ability of the C-lobe to undergo the conformational change to the full open state. In the presence of both Ca2+ and a peptide synthesized to mimic the CaM binding domain on myosin light chain kinase, MLCK-22p, the solution structures of these chimeric CaMs take a similar compact globular shape but their interactions are quite different. The solution structure and interactions of Sc1-72/Gg73-148 CaM are similar to those of Sc1-87/Gg88-148 CaM. The structure of Sc1-87/Gg88-148 CaM is similar to that of Sc1-128/Gg129-148 CaM, but their interactions are different. The result indicates that the replacement of Glu119 with Ala119 has a critical effect on their interactions. Thus, the functional differences among these chimeric CaMs, which have been reported previously [Nakashima, K., et al. (1996) Biochemistry 35, 5602−5610], have been interpreted on the basis of the structures and interactions.