Mapping the Cycle
|The Big Picture: Mapping Mutants to Conformations to Biochemical states|
| Below is a reasonably complete summary
of the genetic information regarding T189I, V192F, L453F, N483S, Y494K and
W501L. Conventions are as follows:
Something truly intriguing has turned up between positions 494 and 501. As portrayed in this graphic, they seem to be twins. Thus the finding of W501R as a suppressor of Y494K (dark gray oval above) leads to the obvious conclusion that W501R confers an opposite effect to W501L. We would therefore predict (more honestly, I DID predict!) that SUPPRESSORS of W501R would be unlike those of W501L; instead, they would be more like 'suppressors of suppressors' of W501L--i.e. akin to suppressors of G680F...
I wuz wrong! the image below depicts a Venn diagram showing the overlap between suppressors of Y494K, W501L, W501R, and W501F and W501Y (these latter two made to investigate whether subtle differences at this position give rise to different spectra of suppressors; this might not have been undertaken had I known that the sweeping change to W501R would have the suppressors it did...):
Some points of note: As indicated by the large amount of overlap, the genetics suggests that the starting mutations (colors on the periphery) are very similar in their effects, in that they can be solved by the same secondary alterations--most notably, the E150K mutation restores function to everybody. The fly in the ointment, however, is W501R--how can this change FIX the Y494K change while simultaneously resembling it? Some models can be found by following this LINK (or will be once I've built this part!)
In the meantime, feast your eyes on THIS: a table of the allowable changes at positions 494/501/691. Note that for positions 494 and 501 (which in most cases behave genetically as twins), as long as we do equal damage to BOTH, things are OK again! Coloring: green is wild type amino acid, red is a change that is mutant when on its own, purple a change that is tolerated, black a change of unknown consequences. Pink background means mutant phenotype, green background means wild-type (or at least much better than the many mutants)
How do we explain this apparently bizarre phenomenon? Relying heavily on discussion and suggestions from Danny Brower and his laboratory, we've created this general model to explain the perplexing findings from the table above: that in general mutating Y494 OR W501 OR G691 results in a mutant phenotype, whereas apparently mutating any TWO gives rise to wild-type phenotype! We also know from Dr. Rayment's crystal structures, as well as those of others, that these three residues are physically associated with one another in at least one state of the myosin motor. To the model!