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
Famous quotes containing the word structure:
“For the structure that we raise,
Time is with materials filled;
Our to-days and yesterdays
Are the blocks with which we build.”
—Henry Wadsworth Longfellow (1809–1882)
“What is the most rigorous law of our being? Growth. No smallest atom of our moral, mental, or physical structure can stand still a year. It grows—it must grow; nothing can prevent it.”
—Mark Twain [Samuel Langhorne Clemens] (1835–1910)
“A structure becomes architectural, and not sculptural, when its elements no longer have their justification in nature.”
—Guillaume Apollinaire (1880–1918)