THE LINKER OF CALMODULIN LACKING GLU84 IS ELONGATED IN SOLUTION, ALTHOUGH IT IS BENT IN THE CRYSTAL

The solution structure of a mutant calmodulin (des84) lacking Glu84 in the central helix linking the two calmodulin lobes is substantially d ifferent from its crystal structure, As determined by small-angle X-ra y scattering, the radius of gyration and the maximum linear dimension of des84 in the presence of 0.1 mM calcium are 20.8 Angstrom and 62.5 Angstrom, respectively, These respective dimensions are larger than th ose expected from the crystal structure of des84, 18.5 Angstrom and 55 .0 Angstrom, and smaller than those expected from the crystal structur e of wild type, 22.8 Angstrom and 67.5 Angstrom. The distance distribu tion function of des84 indicates that it assumes an elongated, dumbbel l shape in solution. The solution scattering profile of des84 is indis tinguishable from that of wild-type calmodulin. The calcium-dependent binding of melittin to des84 causes a change in its shape from elongat ed to spherical, as seen with other calmodulins, In comparison with ca lcium-saturated des84, calcium-free des84 is slightly elongated; a sli ght compaction is observed with native calmodulin. The observed differ ences between the averaged solution structure and the crystal structur e of des84 suggests that an ensemble of structures is available to cal modulin in solution and that its target need not induce a change in it s conformation. These results support the theory that the linker regio n of the central helix of calmodulin functions as a flexible tether. ( C) 1996 Wiley-Liss, Inc.