Thursday, December 17, 2009

a three-billion-year-old patch-up

[deutsche Version]

One of the areas of science that don't get any coverage in the general media concerns the evolution of the protein synthesis apparatus, which in my opinion holds the (as yet hidden) clue to the origin of the protein/RNA/DNA life we know today. It's incredibly important for our fundamental understanding of biology, but maybe it's too complicated to explain to lay people, so nobody bothers.

Paul Schimmel's work on tRNA synthesases (the enzymes that attach amino acids to transfer RNAs so they can be correctly incorporated into proteins) is a prime example. It's a highly complex system that has been around since the time of the last common ancestor of all living things (LUCA), but before that it clearly evolved out of a much simpler system, and understanding the details could open a window into the time before LUCA.

Now Schimmel's group has published in last week's issue of Nature (but without any accompanying fanfare on the front pages) an intriguing example of how evolution dealt with an severe weakness of the system. The tRNA synthetase for alanine (AlaRS) is prone to accept wrong amino acids, especially both the smaller glycine and the larger serine.

Obviously, all "attempts" to fix this by modifying the recognition site have failed, because the key residue in the recognition site is unchanged in all species anybody ever looked at. Having failed to solve the problem, nature developed a patch-up, in the shape of a second enzyme, derived from AlaRS, but working independently, in a secondary check-up stage. This enzyme, known as AlaXp (not sure what the Xp stands for, maybe the need for a patch reminded somebody of Windows?), is so widely distributed today that it also must have been around since the days of LUCA. In a whole series of crystal structures, Schimmel's group has now unraveled how this patch works.

So this, as well as the evolution of the tRNA synthetases, tells us something about how evolution developed the protein synthesis machinery that is universally used today, and specifically how it dealt with a problem that didn't yield to conventional improvements by mutation. But try explaining that to a newspaper editor ...


Reference:

Min Guo, Yeeting E. Chong, Ryan Shapiro, Kirk Beebe, Xiang-Lei Yang & Paul Schimmel
Nature 2009, 462, 808

Paradox of mistranslation of serine for alanine caused by AlaRS recognition dilemma

Alanyl-tRNA synthetases (AlaRSs) may confuse glycine or serine with alanine, potentially causing mistranslation and thus profound functional consequences, with serine posing a bigger challenge than glycine. AlaXps — free-standing, genome-encoded editing proteins — represent one editing checkpoint to prevent this from occurring. Nine crystal structures, together with kinetic and mutational analysis, now show how AlaXps solve the serine misactivation problem.

full text (restricted access)

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