The evidence for G-protein-coupled receptors and heterotrimeric G proteins in protozoa and ancestral metazoa

In higher eukaryotes G-protein-coupled signal transduction pathways are a common mechanism used to detect an extracellular message and transmit a signal, via a membrane-bound receptor and a heterotrimeric G protein, to second messenger producing enzymes and effector proteins. The techniques used to identify components of these pathways are increasingly being applied to protozoa and ancestral metazoa. Many of the organisms studied do seem to express functional homologues of those found in higher eukaryotes and increasingly genes encoding these proteins are being cloned. Sequence analysis of the isolated α-subunits of heterotrimeric G proteins shows that these proteins have extensive homology to their mammalian counterparts, and often show absolute sequence identity in functionally significant regions. The receptor clones isolated clearly establish that protozoa and early metazoa express proteins with seven transmembrane spanning domains. Comparisons with mammalian receptors indicate that these proteins are likely to be regulated by phosphorylation and dephosphorylation events, although the pathways which control these are yet to be identified. The postulated regulatory mechanisms and the number of homologous clones isolated from some protozoa suggest that a highly regulated system of transmembrane signalling appeared at a relatively early stage in evolution.