The effects of calcium site occupancy and reagent length on reactivity of calmodulin lysyl residues with heterobifunctional aryl azides. Mapping interaction domains with specific calmodulin photoprobe derivatives

The relationship of structural and functional moieties on calmodulin is important in all venues of cell activity. In this study, we investigate the effect of lysine modification on calmodulin function. Azidosalicylate reagents containing different 'linker arm' lengths, between the photoactive terminus and an amine-reactive N-hydroxysuccinimidyl ester moiety were used to modify calmodulin lysines at three different positions in a calcium- dependent manner. The short cross-linker, (ASNE-2 (where ASNE represents azidosalicylate N-hydroxysuccinimidyl ester), modifies Lys-75, whereas the longer reagent, ASNE-6, modifies lysines 21, 75, and 94. The modification of these different lysines is shown to be calcium-dependent. At 1-100 μM levels of calcium, only Lys-94 is modified, suggesting that modification of this residue is directed by both the binding of calcium to calcium-binding loops III and IV and the hydrophobic pocket exposed between these two loops as a result of calcium binding. At higher calcium concentrations (>200 μM), where sites I and II become filled, modification of Lys-21 or Lys-75 also was observed. All the modified calmodulins were able to stimulate 3',5'-cyclic- nucleotide phosphodiesterase fully although the K(act) for the Lys-75 and Lys-21 derivatives increased 10- and 50-fold, respectively. None of the modifications affected the activation of erythrocyte plasma membrane Ca²⁺- ATPase. Only the ASNE-6 Lys-75 derivative showed efficient (40%) photocross- linking to the Ca²⁺-ATPase. The ASNE-2 Lys-75 derivative as well as the ASNE-6 Lys-21 and Lys-94 derivatives did not show efficient calcium-dependent photocross-linking to this enzyme.