Calbindin D28k EF-hand ligand binding and oligomerization:: Four high-affinity sites-three modes of action

Calbindin D-28k, a highly conserved protein with Ca2+-sensing and Ca2+-buffering capabilities, is abundant in brain and sensory neurons. This protein contains six EF-hand subdomains, four of which bind Call with high affinity. Calbindin D28k can be reconstituted from six synthetic peptides corresponding to the six EF-hands, indicating a single-domain structure with multiple interactions between the EF-hand subdomains. In this study, we have undertaken a detailed characterization of the Ca2+-binding and oligomerization properties of each individual EF-hand peptide using CD spectroscopy and analytical ultracentrifugation. Under the conditions tested, EF2 is monomeric and does not bind Ca2+, whereas EF6, which binds Call weakly, aggregates severely. We have therefore focused this study on the high-affinity binding sites, EF-hands 1, 3, 4, and 5. Our sedimentation equilibrium data show that, in the presence of Call, EF-hands 1, 3, 4, and 5 all form dimers in solution in which the distribution between the monomer, dimer, and higher order oligomers differs. The processes of Call binding and oligomerization are linked to different degrees, and three main mechanisms emerge. For EF-hands 1 and 5, the dimer binds Ca2+ more strongly than the monomer and Call binding drives dimerization. For EF-hand 4, dimer formation requires only one of the monomers to be Ca2+-bound. In this case, the Call affinity is independent of dimerization. For EF-hand 3, dimerization occurs both in the absence and presence of Call, while oligomerization increases in the presence of Ca2+.