On the origin of symmetry in biology: Protein oligomers

By Charlie Strauss, LANL

Sep 9, 2008

CNLS Conference room.

A variety of surveys have noted that proteins are seldom found natively as monomers and the most common oligomer is the symmetric homodimer. Most previous hypotheses on the origin of this symmetry have postulated various reasons why symmetry is advantageous to an organism and then assumed given an advantage evolution has gradually selected for it. The conundrum with arguments about history is that it is generally impossible to quantitate to what extent any given factor matters.

Instead, we propose a pre-evolutionary hypothesis: The proteins we observe to day are biased towards symmetry, not because symmetry itself has a function per se, but rather because of the primordial pool of all proteins viable for having any function is overwhelmingly symmetric. At first this assertion is surprising because their are vastly more ways to combine two identical proteins asymmetrically than symmetrically or even quasi-symmetrically, however this bias reverses when we restrict attention to the subset of proteins with the minimal requirement for selection.

We have developed a very simple analytic model, confirmed its predictions by detailed computer simulation of protein docking, observe it predicts the known data, and shown quantitatively that this model alone is sufficient to account for the observed bias for symmetry. Our conclusion is that while many factors of course have influenced the precise selection for the symmetric forms observed to day, and indeed many functions even rely on symmetry, that even in the absence of those factors we would expect functional proteins to be predominantly symmetric homomers and specifically homodimers.

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