G protein-coupled receptors (GPCRs) constitute a vast protein family that encompasses a wide range of functions (including various autocrine, para-crine and endocrine processes). They show considerable diversity at the sequence level, on the basis of which they can be separated into distinct groups. We use the term clan to describe the GPCRs, as they embrace a group of families for which there are indications of evolutionary relationship, but between which there is no statistically significant similarity in sequence []. The currently known clan members include the rhodopsin-like GPCRs, the secretin-like GPCRs, the cAMP receptors, the fungal mating pheromone receptors, and the metabotropic glutamate receptor family. The rhodopsin-like GPCRs themselves represent a widespread protein family that includes hormone, neurotransmitter and light receptors, all of which transduce extracellular signals through interaction with guanine nucleotide-binding (G) proteins. Although their activating ligands vary widely in structure and character, the amino acid sequences of the receptors are very similar and are believed to adopt a common structural framework comprising 7 transmembrane (TM) helices [, , ]. A cluster of four intronless GPCR genes, sharing significant sequence similarity with one another, have been identified on human chromosome 19q13.1, downstream from the CD22 gene []. The receptors have been named GPR40, GPR41, GPR42 and GPR43. The GPR42 protein sequence shares more than 98% amino acid identity with GPR41 and is located on a possible polymorphic insert[].
Lyn is a member of the Src subfamily of proteins, which are cytoplasmic (or non-receptor) PTKs. Lyn is expressed in B lymphocytes and myeloid cells. It exhibits both positive and negative regulatory roles in B cell receptor (BCR) signaling. Lyn, as well as Fyn and Blk, promotes B cell activation by phosphorylating ITAMs (immunoreceptor tyr activation motifs) in CD19 and in Ig components of BCR []. It negatively regulates signaling by its unique ability to phosphorylate ITIMs (immunoreceptor tyr inhibition motifs) in cell surface receptors like CD22 and CD5 []. Lyn also plays an important role in G-CSF receptor signaling by phosphorylating a variety of adaptor molecules []. Src kinases contain an N-terminal SH4 domain with a myristoylation site, followed by SH3 and SH2 domains, a tyr kinase domain, and a regulatory C-terminal region containing a conserved tyr. They are activated by autophosphorylation at the tyr kinase domain, but are negatively regulated by phosphorylation at the C-terminal tyr by Csk (C-terminal Src Kinase). The SH3 domain of Src kinases contributes to substrate recruitment by binding adaptor proteins/substrates, and regulation of kinase activity through an intramolecular interaction [, ].
G protein-coupled receptors (GPCRs) constitute a vast protein family that encompasses a wide range of functions (including various autocrine, paracrine and endocrine processes). They show considerable diversity at the sequence level, on the basis of which they can be separated into distinct groups. We use the term clan to describe the GPCRs, as they embrace a group of families for which there are indications of evolutionary relationship, but between which there is no statistically significant similarity in sequence []. The currently known clan members include the rhodopsin-like GPCRs, the secretin-like GPCRs, the cAMP receptors, the fungal mating pheromone receptors, and the metabotropic glutamate receptor family. The rhodopsin-like GPCRs themselves represent a widespread protein family that includes hormone, neurotransmitter and light receptors, all of which transduce extracellular signals through interaction with guanine nucleotide-binding (G) proteins. Although their activating ligands vary widely in structure and character, the amino acid sequences of the receptors are very similar and are believed to adopt a common structural framework comprising 7 transmembrane (TM) helices [, , ]. A cluster of four intronless GPCR genes, sharing significant sequence similarity with one another, have been identified on human chromosome 19q13.1, downstream from the CD22 gene []. The receptors have been named GPR40, GPR41, GPR42 and GPR43. The GPR42 protein sequence shares more than 98% amino acid identity with GPR41 and is located on a possible polymorphic insert []. GPR40 has recently been shown to bind long-chain free fatty acids, molecules that have a role in various cellular processes, including regulation of insulin secretion [, ]. Expression of GPR40 is restricted to the pancreas, with high levels in the islets and pancreatic beta cell lines []. Upon activation, GPR40 appears to couple predominantly to Gq and partially to Gi proteins, and has been shown to amplify glucose-stimulated insulin secretion from pancreatic beta cells. The receptor may therefore be a potential target for anti-diabetic drugs [].
