Structure
Ras contains a six-stranded beta sheet and 5 alpha helices:
- G domain (166 amino acids) which binds guanosine nucleotides, about 20kDa.
- C terminal membrane targeting region (CAAX-COOH, also known as CAAX box) which is lipid-modified by farnesyl transferase, RCE1 and ICMT
The G domain contains five G motifs that bind GDP/GTP directly
- G1 - P-loop binds the beta phosphate of GDP and GTP
- G2 - threonine-35 also switch 1, binds the terminal phosphate of GTP, but makes no contacts with GDP
- G3 - DXXG motif, aspartate-57 is specific for guanine rather than adenine
- G4 - LVGNKxDL motif
- G5 - SAK consensus sequence, the alanine-146 is specific for guanine rather than adenine
and two switches which are the main parts of the protein that move upon activation by GTP.
- switch I includes threonine-35
- switch II glycine-60 in DXXG motif
Ras also binds a magnesium ion which helps to coordinate nucleotide binding.
Read more about this topic: Ras Subfamily
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