This entry includes F-box only protein 30 (Fbxo30) []and F-box only protein 40 (Fbxo40) []. Fbxo30 is a ubiquitin ligase required for muscle loss, hence it is also known as muscle ubiquitin ligase of the SCF complex in atrophy-1 (MUSA1) []. Fbxo40 id an E3 ubiquitin ligase that induces IRS1 ubiquitination and breakdown specifically in skeletal muscle cells and only upon IGF1 (insulin-like growth factor 1) stimulation [].
Proteins in this entry consist of a Ras-associated (RA) domain, a PH domain, a family-specific BPS region, and a C-terminal SH2 domain. Grb7, Grb10 and Grb14 are paralogues that are also present in this entry []. These adapter proteins bind a variety of receptor tyrosine kinases, including the insulin and insulin-like growth factor-1 (IGF1) receptors. Grb10 and Grb14 are important tissue-specific negative regulators of insulin and IGF1 signaling based and may contribute to type 2 (non-insulin-dependent) diabetes in humans. RA-PH function as a single structural unit and is dimerized via a helical extension of the PH domain. The PH domain here are proposed to bind phosphoinositides non-cannonically and are unlikely to bind an activated GTPase []. This entry also includes APBB1IP (also known as RIAM), which functions in the signal transduction from Ras activation to actin cytoskeletal remodelling [].
The insulin family of proteins groups together several evolutionarily related active peptides []: these include insulin [, ], relaxin [, ], insect prothoracicotropic hormone (bombyxin) [], insulin-like growth factors (IGF1 and IGF2) [, ], mammalian Leydig cell-specific insulin-like peptide (gene INSL3), early placenta insulin-like peptide (ELIP) (gene INSL4), locust insulin-related peptide (LIRP), molluscan insulin-related peptides (MIP) and Caenorhabditis elegans insulin-like peptides. The 3D structures of a number of family members have been determined [, , ]. The fold comprises two polypeptide chains (A and B) linked by two disulphide bonds: all share a conserved arrangement of 4 cysteines in their A chain, the first of which is linked by a disulphide bond to the third, while the second and fourth are linked by interchain disulphide bonds to cysteines in the B chain. The IGFs, or somatomedins, play a key role in pre-adolescent mammalian growth. IGFI expression is regulated by growth hormone and mediates post-natal growth []. Defects in IGF1 are the cause of insulin-like growth factor I deficiency (IGF1 deficiency), an autosomal recessive disorder characterised by growth retardation, sensorineural deafness and mental retardation []. IGF2 appears to be induced by placental lactogen and is thought to play a role in foetal development [].
The insulin family of proteins groups together several evolutionarily related active peptides []: these include insulin [, ], relaxin [, ], insect prothoracicotropic hormone (bombyxin) [], insulin-like growth factors (IGF1 and IGF2) [, ], mammalian Leydig cell-specific insulin-like peptide (gene INSL3), early placenta insulin-like peptide (ELIP) (gene INSL4), locust insulin-related peptide (LIRP), molluscan insulin-related peptides (MIP) and Caenorhabditis elegans insulin-like peptides. The 3D structures of a number of family members have been determined [, , ]. The fold comprises two polypeptide chains (A and B) linked by two disulphide bonds: all share a conserved arrangement of 4 cysteines in their A chain, the first of which is linked by a disulphide bond to the third, while the second and fourth are linked by interchain disulphide bonds to cysteines in the B chain. The IGFs, or somatomedins, play a key role in pre-adolescent mammalian growth. IGFI expression is regulated by growth hormone and mediates post-natal growth []. Defects in IGF1 are the cause of insulin-like growth factor I deficiency (IGF1 deficiency), an autosomal recessive disorder characterised by growth retardation, sensorineural deafness and mental retardation [].
Amyloid beta A4 precursor protein-binding family B member 1-interacting protein (APBB1IP) consists of a Ras-associated (RA) domain, a PH domain, a family-specific BPS region, and a C-terminal SH2 domain. Grb7, Grb10 and Grb14 are paralogues that are also present in this entry []. These adapter proteins bind a variety of receptor tyrosine kinases, including the insulin and insulin-like growth factor-1 (IGF1) receptors. Grb10 and Grb14 are important tissue-specific negative regulators of insulin and IGF1 signaling based and may contribute to type 2 (non-insulin-dependent) diabetes in humans. RA-PH function as a single structural unit and is dimerized via a helical extension of the PH domain. The PH domain here are proposed to bind phosphoinositides non-cannonically and are unlikely to bind an activated GTPase []. The tandem RA-PH domains are present in a second adapter-protein family, MRL proteins, Caenorhabditis elegansprotein MIG-1012, the mammalian proteins RIAM and lamellipodin and the Drosophila melanogasterprotein Pico12, all of which are Ena/VASP-binding proteins involved in actin-cytoskeleton rearrangement.PH domains have diverse functions, but in general are involved in targeting proteins to the appropriate cellular location or in the interaction with a binding partner. They share little sequence conservation, but all have a common fold, which is electrostatically polarized. Less than 10% of PH domains bind phosphoinositide phosphates (PIPs) with high affinity and specificity. PH domains are distinguished from other PIP-binding domains by their specific high-affinity binding to PIPs with two vicinal phosphate groups: PtdIns(3,4)P2, PtdIns(4,5)P2 or PtdIns(3,4,5)P3 which results in targeting some PH domain proteins to the plasma membrane []. A few display strong specificity in lipid binding. Any specificity is usually determined by loop regions or insertions in the N terminus of the domain, which are not conserved across all PH domains. PH domains are found in cellular signaling proteins such as serine/threonine kinase, tyrosine kinases, regulators of G-proteins, endocytotic GTPases, adaptors, as well as cytoskeletal associated molecules and in lipid associated enzymes.
Protein phosphorylation, which plays a key role in most cellular activities, is a reversible process mediated by protein kinases and phosphoprotein phosphatases. Protein kinases catalyse the transfer of the gamma phosphate from nucleotide triphosphates (often ATP) to one or more amino acid residues in a protein substrate side chain, resulting in a conformational change affecting protein function. Phosphoprotein phosphatases catalyse the reverse process. Protein kinases fall into three broad classes, characterised with respect to substrate specificity []:Serine/threonine-protein kinasesTyrosine-protein kinasesDual specificity protein kinases (e.g. MEK - phosphorylates both Thr and Tyr on target proteins)Protein kinase function is evolutionarily conserved from Escherichia coli to human []. Protein kinases play a role in a multitude of cellular processes, including division, proliferation, apoptosis, and differentiation []. Phosphorylation usually results in a functional change of the target protein by changing enzyme activity, cellular location, or association with other proteins. The catalytic subunits of protein kinases are highly conserved, and several structures have been solved [], leading to large screens to develop kinase-specific inhibitors for the treatments of a number of diseases [].Tyrosine-protein kinases can transfer a phosphate group from ATP to a tyrosine residue in a protein. These enzymes can be divided into two main groups []:Receptor tyrosine kinases (RTK), which are transmembrane proteins involved in signal transduction; they play key roles in growth, differentiation, metabolism, adhesion, motility, death and oncogenesis []. RTKs are composed of 3 domains: an extracellular domain (binds ligand), a transmembrane (TM) domain, and an intracellular catalytic domain (phosphorylates substrate). The TM domain plays an important role in the dimerisation process necessary for signal transduction []. Cytoplasmic / non-receptor tyrosine kinases, which act as regulatory proteins, playing key roles in cell differentiation, motility, proliferation, and survival. For example, the Src-family of protein-tyrosine kinases [].This entry represents the insulin receptor, as well as related insulin-like receptors. The insulin receptor binds insulin and has a tyrosine-protein kinase activity, and mediates the metabolic functions of insulin. Binding to insulin stimulates the association of the receptor with downstream mediators, including IRS1 and phosphatidylinositol 3'-kinase (PI3K). The insulin receptor can activate PI3K either directly by binding to the p85 regulatory subunit, or indirectly via IRS1. When the insulin receptor is present in a hybrid receptor with IGF1R (insulin growth factor receptor), it binds IGF1 (insulin growth factor 1) [, , ].
