Cloning and sequence of multiple forms of phospholipase C

Three phospholipase C isozymes (PLC-I, II, and III) have been purified from bovine brain. Here, phospholipase C-related cDNA clones corresponding to PLC-I and PLC-III were isolated from a rat brain λgt11 expression cDNA library using specific monoclonal antibodies and sequenced. Each of them encodes a distinct polypeptide with a calculated molecular mass of 138,225 (PLC-I) and 85,840 (PLC-III). Comparison of these two with the sequence of another isozyme PLC-II (M r = 148,431) that we have previously characterized revealed a low overall sequence homology. Nevertheless, a significant amino acid sequence similarity between the three enzymes was found in two regions, one of about 150 amino acids and the other of about 120 amino acids. The two conserved domains were separated by a variable region. The variable region sequence of PLC-II is relatively long and has recently been shown to contain regions homologous to the noncatalytic domain of the nonreceptor tyrosine kinases. Those of PLC-I and III were short and appeared to be unrelated to these tyrosine kinases. The physiological implications of the multiple species of PLC enzymes are discussed.