Dissection of calbindin D9k into two Ca(2+)-binding subdomains by a combination of mutagenesis and chemical cleavage.

Calbindin D9k is a 75-residue globular protein made up of two Ca2+ binding subdomains of the EF-hand type. In order to examine the subdomains independently, a method was devised to selectively cleave the loop between them. Using site-directed mutagenesis, a unique methionine was substituted for Pro43 in the loop, thus allowing cleavage using cyanogen bromide. Agarose gel electrophoresis shows that the fragments have a high affinity for one another, although less so in the absence of calcium. 1H-NMR spectra of the fragments indicate that the structures of the heterodimers are changed little from that of the intact protein. However, the Ca2+ binding constants of the individual subdomains are several orders of magnitude lower than for the corresponding sites in the uncleaved protein.