Everything about Argonaute totally explained
Argonaute proteins are the
catalytic components of the
RNA-induced silencing complex (RISC), the
protein complex responsible for the
gene silencing phenomenon known as
RNA interference (RNAi). Argonaute proteins bind
small interfering RNA (siRNA) fragments and have
endonuclease activity directed against
messenger RNA (mRNA) strands that are
complementary to their bound siRNA fragment. The proteins are also partially responsible for selection of the guide strand and destruction of the passenger strand of the siRNA substrate.
The structural basis for binding of RNA to the argonaute protein has been examined by
X-ray crystallography of the binding
domain of an RNA-bound argonaute protein. The
phosphorylated 5' end of the RNA strand enters a
conserved basic surface
pocket and makes contacts through a
divalent cation such as
magnesium and by
aromatic stacking between the 5' nucleotide in the siRNA and a conserved
tyrosine residue. This site is thought to form a nucleation site for the binding of the siRNA to its mRNA target.
In
eukaryotes, argonaute proteins have been identified in high concentrations in regions of the cell's
cytoplasm known as cytoplasmic bodies, to which mRNA decay is also localized. The argonaute protein family is shared among not only eukaryotes, but also
archaea and certain
bacteria such as
Aquifex aeolicus. Based on
comparative genomics studies, the argonaute family is thought to have
evolved from components of the
translation initiation system.
Argonaute proteins are named after the
argonaute (AGO) phenotype of
Arabidopsis mutants, which itself was named after its resemblance to
argonauts.
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