RasGRF1 (also called GRF1; CDC25Mm/Ras-specific nucleotide exchange factor CDC25; GNRP/Guanine nucleotide-releasing protein) and RasGRF2 (also called GRF2; Ras guanine nucleotide exchange factor 2) are a family of guanine nucleotide exchange factors (GEFs). They both promote the exchange of Ras-bound GDP by GTP, thereby regulating the RAS signaling pathway. RasGRF1 and RasGRF2 form homooligomers and heterooligomers. GRF1 has 3 isoforms and GRF2 has 2 isoforms. The longest isoforms of RasGRF1 and RasGRF2 contain the following domains: a Rho-GEF domain sandwiched between 2 PH domains, IQ domains, a REM (Ras exchanger motif) domain, and a Ras-GEF domainwhich gives them the capacity to activate both Ras and Rac GTPases in response to signals from a variety of neurotransmitter receptors. Their IQ domains allow them to act as calcium sensors to mediate the actions of NMDA-type and calcium-permeable AMPA-type glutamate receptors. GRF1 also mediates the action of dopamine receptors that signal through cAMP. GRF1 and GRF2 play strikingly different roles in regulating MAP kinase family members, neuronal synaptic plasticity, specific forms of learning and memory, and behavioral responses to psychoactive drugs. 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.
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