Phospholipase C epsilon: a novel Ras effector

Three classes of mammalian phosphoinositide-specific phospholipase C (PLC) have been characterized, PLC beta, PLC gamma and PLC delta, that are differ entially regulated by heterotrimeric G-proteins, tyrosine kinases and calci um. Here we describe a fourth class, PLC epsilon, that in addition to conse rved PLC domains, contains a GTP exchange factor (GRF CDC25) domain and two C-terminal Ras-binding (RA) domains, RA1 and RA2, The RA2 domain binds H-R as in a GTP-dependent manner, comparable with the Ras-binding domain of Raf -1; however, the RA1 domain binds H-Ras with a low affinity in a GTP-indepe ndent manner. While G alpha (q), G beta gamma or, surprisingly, H-Ras do no t activate recombinant purified protein in vitro, constitutively active Q61 L H-Ras stimulates PLC epsilon co-expressed in COS-7 cells in parallel with Ras binding. Deletion of either the RA1 or RA2 domain inhibits this activa tion. Site-directed mutagenesis of the RA2 domain or Ras demonstrates a con served Ras-effector interaction and a unique profile of activation by Ras e ffector domain mutants, These studies identify a novel fourth class of mamm alian PLC that is directly regulated by Ras and links two critical signalin g pathways.