The Evidence for G-Protein-Coupled Receptors and Heterotrimeric G Proteins in Protozoa and Ancestral Metazoa

Abstract

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.

Molecular Machinations: Chemokine Signals in Host-Pathogen Interactions

“…Indeed, G-protein-mediated migratory responses are demonstrable in protozoa (126). Comparative amino acid sequence analysis of mammals and primitive vertebrates indicate that the CXC and CC groups of chemokines diverged from an ancestral gene even before the divergence of the various orders of mammals and probably before the emergence of vertebrates (78,124).…”

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“…Heterotrimeric G proteins (G proteins) are well characterized in mammals and yeast (New and Wong 1998), however, much less is known about G proteins in other eukaryotes such as plants. G proteins consist of , and subunits and associate with a family of plasma membrane receptors known as G protein coupled receptors (GPCRs).…”

The secretin G-protein-coupled receptor family: teleost receptors

“…The specific properties of the various receptors are conferred by their N-terminal extracellular and C-terminal intracellular domains, and also their intracellular loops. They are among the oldest devices devoted to signal transduction, having been identified in plants, protozoa and the earliest diploblastic metazoa (Vernier et al 1995, New & Wong 1998, Plakidou-Dymock et al 1998.…”