The 150-kDa phospholipase C (PLC)-beta 1 and three immunologically related proteins with molecular sizes of 140, 100, and 45 kDa were purified from bovine brain extracts. Determination of the amino-terminal amino acid sequence of the 45-kDa protein and immunoblots of the purified proteins with sequence-specific antibodies to peptides corresponding to three different regions of PLC-beta 1 suggest that a single cleavage at the linkage between amino acid residues 880 and 881 of PLC-beta 1 generates the 100- and 45-kDa proteins, which correspond to the amino-terminal and carboxyl-terminal portions, respectively, of PLC-beta 1. The Ca(2+)-dependent protease calpain appears to be responsible for the cleavage of PLC-beta 1; the PLC-beta 1 amino acid sequence contains PEST sequences which are common to proteins susceptible to calpain, and limited proteolysis of purified PLC-beta 1 by calpain generated a 100-kDa protein and a 40-kDa protein that contains the same amino-terminal sequence as the 45-kDa protein. The 140-kDa protein lacks the carboxyl-terminal-most region of PLC-beta 1, but there is no evidence it is derived from PLC-beta 1 by proteolysis. Cleavage of PLC-beta 1 by calpain had no significant effect on catalytic activity measured in the absence of the alpha subunit of the G alpha q but completely abrogated the stimulatory effect of G alpha q. On the other hand, G alpha q activated the 140-kDa enzyme. These results suggest that the region between residue 881 and the most carboxyl-terminal 10 kDa of PLC-beta 1 contains the G alpha q interaction site.