A calcium-binding protein from rabbit lung cytosol identified as the product of growth-regulated gene (2A9) and its binding proteins

Using Ca²⁺-dependent affinity chromatography on a synthetic compound (W-77)-coupled Sepharose 4B column, we purified two different Ca²⁺-binding proteins from rabbit lung extracts. The molecular weights of these proteins were estimated to be 17 kDa (calmodulin) and 10 kDa, respectively. The partial amino acid sequence of the 10-kDa protein revealed that it has two EF-hand structures. In addition, the 10-kDa protein was highly homologous (91%) to the product of growth-regulated gene, 2A9 (calcyclin). The Ca²⁺-binding property of the 10-kDa protein was observed by a change in the uv difference spectrum. Equilibrium dialysis showed that 1 mol of the 10-kDa protein bound to 2.04 ± 0.05 mol of Ca²⁺ in the presence of 10^-4 m Ca²⁺. However, the protein failed to activate calmodulin-dependent enzymes such as Ca²⁺ CaM kinase II, myosin light chain kinase, and phosphodiesterase. We found that a 50-kDa cytosolic protein of the rabbit lung, intestine, and spleen bound to the 10-kDa protein, in a Ca²⁺-dependent manner. The distribution of calcyclin and calcyclin binding proteins was unique and seems to differ from that of calmodulin and calmodulin-binding proteins. Thus, calcyclin probably plays a physiological role through its binding proteins for the Ca²⁺-dependent cellular response.