Tuesday, June 15, 2010

A possible mechanism for elongation factors--finally?

Like the last speculation, this one involves the ribosome. In particular, the two elongation factors that in bacteria are known as EF-Tu and EF-G. Like all GTPases, these proteins have a catalytic domain that resembles the Ras family of signaling molecules. Clearly, however, the details of the catalysis are distinct. In common with each other, both types of proteins have low catalytic activity on their own (though the quantitative meaning of "low" varies"), and need to be activated by binding to something else in order to achieve rapid catalysis.

In the case of Ras-like proteins, these partners are called GTPase-activating proteins, or GAPs, and they bind to a particular region of the proteins near the phosphates of the bound GTP. Their activity commonly (though not always) involves an arginine side chain that is inserted into the active site and presumably stabilizes the transition state. However, EF-Tu and EF-G are activated by interacting with a particular state of the ribosome. The sarcin-ricin loop, or SRL, of the large subunit occupies a position similar to GAPs in Ras-like GTPases, but in the available structures it makes few contacts to the protein. This is probably due to the fact that one part of the protein, the so-called Switch I segment, is disordered.