Calcium-binding calmyrin forms stable covalent dimers in vitro, but in vivo is found in monomeric form

The EF-hand Ca(2+)-binding protein calmyrin is expressed in many tissues and can interact with multiple effector proteins, probably as a sensor transferring Ca(2+) signals. As oligomerization may represent one of Ca(2+)-signal transduction mechanisms, we characterised recombinant calmyrin forms using non-reducing SDS/PAGE, analytical ultracentrifugation and gel filtration. We also aimed at identification of biologically active calmyrin forms. Non-reducing SDS/PAGE showed that in vitro apo- and Ca(2+)-bound calmyrin oligomerizes forming stable intermolecular disulfide bridges. Ultracentrifugation indicated that at a 220 microM initial protein concentration apo-calmyrin existed in an equilibrium of a 21.9 kDa monomer and a 43.8 kDa dimer (trimeric or tetrameric species were not detected). The dimerization constant was calculated as Ka = 1.78 x 103 M(-1) at 6 degrees C. Gel filtration of apo- and Ca(2+)-bound calmyrin at a 100 microM protein concentration confirmed an equilibrium of a monomer and a covalent dimer state. Importantly, both monomer and dimer underwent significant conformational changes in response to binding of Ca(2+). However, when calmyrin forms were analyzed under non-reducing conditions in cell extracts by Western blotting, only monomeric calmyrin was detected in human platelets and lymphocytes, and in rat brain. Moreover, in contrast to recombinant calmyrin, crosslinking did not preserve any dimeric species of calmyrin regardless of Ca(2+) concentrations. In summary, our data indicate that although calmyrin forms stable covalent dimers in vitro, it most probably functions as a monomer in vivo.