A novel phospholipase C delta 4 (PLC delta 4) splice variant as a negativeregulator of PLC

It has been reported that there are two alternatively spliced variants of p hospholipase C-delta 4 (PLC delta 4), termed ALT I and II, that contain an additional 32 and 14 amino acids in their respective sequences in the linke r region between the catalytic X and Y domains (Lee, S. B., and Rhee, S. G. (1996) J. Biol. Chem. 271, 25-31). We report here the isolation and charac terization of a novel alternative splicing isoform of PLC delta 4, termed A LT III, as a negative regulator of PLC. In ALT III, alternative splicing oc curred in the catalytic X domain, i.e. 63 amino acids (residues 424-486) co ntaining the C-terminal of the X domain and linker region were substituted for 32 amino acids corresponding to the insert sequence of ALT I. Although the expression level of ALT III was found to be much lower in most tissues and cells compared with that of PLC delta 4, it was significantly higher in some neural cells, such as NIE-115 cells and p19 cells differentiated to n eural cells by retinoic acid. Interestingly, recombinant ALT III protein di d not retain enzymatic activity, and the activity of PLC delta 4 overexpres sed in COS7 cells was markedly decreased by the co-expression of ALT III bu t not by ALT I or II. Moreover, N-terminal pleckstrin homology domain (PH d omain) of ALT III alone could inhibit the increase of inositol-1,4,5-trisph osphate levels in PLC delta 4-overexpressing NIH3T3 cells, whereas a PH dom ain deletion mutant could not, indicating that the PH domain is necessary a nd sufficient for its inhibitory effect. The ALT III PH domain specifically bound to phosphatidylinositol (PtdIns)-4,5-P-2 and PtdIns-3,4,5-P-3 but no t PtdIns, PtdIns-4-P, or inositol phosphates, and the mutant R36G, which re tained only weak affinity for PtdIns-4,5-P-2, could not inhibit the activit y of PLC delta 4. These results indicate that PtdIns-4,5-P-2 binding to PH domain is essential for the inhibitory effect of ALT III. ALT III also inhi bited PLC delta 1 activity and partially suppressed PLC gamma 1 activity, b ut not PLC beta 1 in vitro; it did inhibit all types of isozymes tested in vivo. Taken together, our results indicate that ALT III is a negative regul ator of PLC that is most effective against the PLC delta-type isozymes, and its PH domain is essential for its function.