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Cover Illustration: A fragment of Calmodulin (CaM) composed of EF-hands 2 and 3 and the intervening linker sequence (CaM2/3) adopts a Ca²⁺-dependent structure remarkably similar to both the N- and C-domains of Ca²⁺-ligated CaM. Shown here is the superimposed C^α backbone trace of CaM2/3 (Ca²⁺-binding segments blue; helices red; β-sheets cyan; linker yellow; disordered termini magenta) on the N-domain of Ca²⁺-free CaM (transparent green) and Ca²⁺-ligated CaM (transparent gray). In the case of Ca²⁺-ligated CaM, stacking interactions between four conserved aromatic residues hold the first and fourth helices of each EFhand domain together, while the loop between EF-hands covalently tethers the second and third helices. This may help position the “odd-even” paired EF-hands for high affinity, cooperative Ca²⁺-binding. In contrast, these aromatic residues lie along the second and third helices of CaM2/3, and thus are adjacent to the loop between its “even-odd” paired EF-hands. Such nonnative hydrophobic core packing may contribute to the low affinity and sequential Ca²⁺-binding exhibited by CaM2/3. (For details, see Lakowski et al., pp. 1119–1132.)