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Growth factor receptor-bound protein 2 also known as Grb2, Adapter protein GRB2, Protein Ash, or SH2/SH3 adapter GRB2 is an intracellular adaptor protein involved in growth-factor mediated signal transduction and cell communication.

This protein is encoded by the GRB2 gene or ASH gene and is expressed in all tissues throughout mammalian development and essential for multiple cellular functions.

GRB2 acts as an internal receptor triggering downstream signals in many cell types. The main example of GRB2 is seen in activation of the Ras pathway through association with Son of Sevenless or Sos1 and 2, a guanine nucleotide exchange factor. Activation of this pathway leads to subsequent signalling cascades to be triggered and ultimately, gene expression.

GRB2 is the mammalian homologue to Sem-5, a similar adaptor protein found in C. elegans involved in vulval development and sex myoblast migration and also Drk, found in Drosophila, involved in signal transduction.

Structure
GRB2 protein is 217 AA in length, encoded by the gene GRB2 (also called ASH gene) located on chromosome 17 at position 17q25.1.

The GRB2 gene encodes six transcripts, five of which are protein coding. Two alternatively spliced transcript variants encoding different isoforms have been found for this gene; the GRB2 form and also the GRB3-3 isoform.

The 25kDa protein consists of a SH2 domain flanked by two SH3 domains. Each domain acts specifically; the SH2 domain of GRB2 typically binds phosphorylated tyrosine sequences on receptors or scaffold proteins, with a preference for pY-X-N-X, where X is generally a hydrophobic residue such as valine. The N-terminal SH3 domain forms complex associations with proline-rich regions of other proteins and Sos, whereas the C-terminal SH3 domain binds to peptides conforming to a P-X-I/L/V/-D/N-R-X-X-K-P motif that allows it to specifically bind to proteins such as Gab-1.

Interaction in Ras pathway
Activation of Receptor-linked tyrosine kinases, for example, Epidermal Growth Factor Receptor (EGFR) by a ligand (e.g. Epidermal Growth Factor) results in trans phosphorylation of the receptor's cytoplasmic tyrosine residues located at the C-terminus. The phosphorylated cytoplasmic tails of these receptors recruit GRB2 proteins which dock onto the phosphotyrosine residues via their SH2 domain at the plasma membrane. Guanine nucleotide exchange factor Sos binds GRB2 by forming associations with the two SH3 domains on the GRB2 protein. The formation of the EGFR-GRB2-SOS complex bring Sos into range with RAS and activates Sos - GEF activity; replacing Ras-GDP with GTP, and subsequently activating the Ras signalling pathway. Ras initiates a cascade of downstream signals, such as MAP kinase and ERK1,2, which ultimately leads to the expression of genes involved in cell proliferation, differentiation and survival.

Interaction in TCR signalling
See: Lck

GRB2 is also expressed in lymphocytes where it plays a role in antigen receptor signalling. Activation of a T-cell Receptor (TCR) by a pathogen triggers Lck, a tyrosine kinase, to phosphorylate the associated CD3 complex thereby initiating TCR signalling and subsequently activating all three MAP kinase families. It is thought GRB2 regulates Lck activity through negative feedback, playing a role in negative selection of T cells. Negative selection or T-cell mediated apoptosis, is an important process ensuring a functioning immune system by removing potentially fatal auto-immune T-lymphocyctes, which are seen to have a too high affinity for self-peptide MHC complexes. It is speculated that this process involves GRB2 which downregulates the activity of Lck and thus deletes self-recognising T-cells.

Misexpression
GRB3-3 GRB3-3 is a natural isoform of GRB2 formed as a result of a deletion in the SH2 region of the protein. Unlike GRB2, GRB3-3 does not bind phosphorylated EGFRs (since this isoform lacks the SH2 domain) but instead inhibits EGF-induced Ras-activation in a dominant fashion over GRB2. Lack of Ras pathway activation and subsequently the downstream signalling pathways has been implicated in programmed cell death.

Disease
Human Breast Cancer

Over expression of GRB2 protein has been implicated with tumour progression. Comparison studies between normal breast epithelium and cancerous breast epithelium observed a 2-fold increase in GRB2 mRNA in breast cancer cells, due to amplification of the GRB2 locus. The upregulation of the protein and subsequently the Ras pathway may alter cell growth signalling and thus play a role in tumour formation.

Death

Homozygous knock out: GRB2 has been shown to play a vital role in early embryogenesis. Embryonic studies in mice with homozygous mutations resulting in a GRB2 knock-out (GRB2 -/-)were shown to have reduced cell proliferation and normally died by day 7.5.

Heterozygous knock out: Heterozygous gene knock out experiments for GRB2 result in impaired immune response by altering T-cell development.