cAMP signalling in Trypanosoma brucei

Cyclic AMP was the first second messenger to be identified. After five deca des of research, much is currently known about its biological functions and clinical implications. Several components of the cAMP signalling pathways, such as the G-protein coupled receptors and the phosphodiesterases, have b ecome sensitive and specific drug targets for a host of clinical applicatio ns. Surprisingly, very little effort has been inyested so far into the stud y of cAMP signalling in parasites, and its significance in host/parasite in teraction. Our laboratory has embarked on a study of cAMP signalling in Try panosoma brucei. A newly identified adenylyl cyclase, GRESAG4.4B, a member of a small family of closely related genes, is being used as a model molecu le for investigating the mechanisms which control cyclase activity in the T . brucei cell. On the other hand, a number of genes for different families of cAMP-specific phosphodiesterases have been identified and characterised. One enzyme, TbPDE1, is coded for by a single-copy gene. Knock-outs of this gene display an almost normal phenotype in culture, indicating that TbPDE1 is not an essential enzyme under culture conditions. A second phosphodiest erase which is being studied in detail, TbPDE2A, is clearly different from TbPDE1, and it is coded for by a member of a small gene family containing a bout six similar, but non-identical genes. TbPDE2A, as TbPDE1, is specific for cAMP. In its N-terminal, it contains a GAF domain which may represent a n allosteric cGMP-binding site. The other members of the TbPDE2 family all exhibit strongly conserved catalytic domains, but vary widely in their N-te rminal regulatory domains. With regard to downstream signalling by the cAMP generated through the interplay of adenylyl cyclases and phosphodiesterase s, we have recently identified a single-copy gene (TbRSU1) which codes for a putative regulatory subunit of the cAMP-regulated protein kinase A. This protein exhibits considerable similarity with its mammalian counterparts. I mmunoprecipitation co-precipitates a protein kinase activity with the chara cteristics of protein kinase A. (C) 2001 Australian Society for Parasitolog y Inc. Published by Elsevier Science Ltd. All rights reserved.