Differential expression of calmodulin-binding proteins in B, T lymphocytes and thymocytes

Changes in intracellular free Ca²⁺ are involved in the transmembrane signalling of different cells, including lymphocytes ^1-3. Since calmodulin (CaM) is a primary receptor for Ca²⁺ (ref. 4), it may mediate the activation of crucial enzymes after antigen-induced increases in cytosolic Ca²⁺. Using a biotinylated-CaM (Bio-CaM) detection procedure⁵ to identify such proteins, we found that a peptide of relative molecular mass 59,000 (59K) was the predominant soluble CaM-binding protein (CaM-BP) in T cells and B lymphocytes from murine spleen; immunoblotting experiments identified it as a subunit of the CaM-dependent phosphatase, 'calcineurin' (CN)⁶. Smaller amounts of larger CaM-BPs, thought to be cytoskeletal-binding proteins, were also detected. CaM-BPs were expressed differentially, with B lymphocytes having four times more of the CN-like protein than T lymphocytes, while in thymocytes, a 65K polypeptide was the major CaM-BP. However, limited proteolysis analysis suggested that this thymus-specific peptide may be a precursor of CN. These data suggest that Ca²⁺-stimulated protein dephosphorylation may be an important and highly regulated function in lymphoid cells.