A FAMILY OF PUTATIVE RECEPTOR-ADENYLATE CYCLASES FROM LEISHMANIA-DONOVANI

Leishmania parasites are exposed to pronounced changes in their enviro nment during their life cycle as they migrate from the sandfly midgut to the insect proboscis and then into the phagolysosomes of the verteb rate macrophages. The developmental transformations that produce each life cycle stage of the parasite may be signaled in part by binding of environmental ligands to receptors which mediate transduction of extr acellular signals. We have identified a family of five clustered genes in Leishmania donovani which may encode signal transduction receptors . The coding regions of two of these genes, designated rac-A and rac-B , have been sequenced and shown to code for proteins with an NH2-termi nal hydrophilic domain, an intervening putative transmembrane segment, and a COOH-terminal domain that has high sequence identity to the cat alytic domain from adenylate cyclases in other eukaryotes. We have exp ressed the receptor-adenylate cyclase protein (RAC)-A protein in Xenop us oocytes and demonstrated that it functions as an adenylate cyclase. Although RAC-B exhibits no catalytic activity when expressed in oocyt es, co-expression of RAC-A and RAC-B negatively regulates the adenylat e cyclase activity of RAC-A, suggesting that these two proteins intera ct in the membrane. Furthermore, a truncated version of RAC-A function s as a dominant negative mutant that inhibits the catalytic activity o f the wild type receptor. The rac-A and rac-B genes encode development ally regulated mRNAs which are expressed in the insect stage but not i n the mammalian host stage of the parasite life cycle.