Sea of Hypotheses
Does the Chin Quiver when the Whale Bites?

Who cares about actin anyway?



ne of the key unknowns in myosin research at this point is just what is going on when myosin interacts with actin. Does this interaction drive the stroking motion? How is it coordinated with Pi release? Why is myosin so reluctant to initiate the strong interaction with actin? WHAT DOES IT LOOK LIKE? Ideally, these questions might be answered with x-ray crystallography, but for technical reasons, this is currently not a possibility.




o what can genetics, phylogenetics and rampant conjecture do for us? Hopefully, these techniques, in conjunction with the crystal structures we do have, will map out the mystery states of myosin. Several pieces of the puzzle are currently in hand; it may well be only a matter of trying to fit them together into a coherent whole.

 

LandMarks: What can we see?


o misquote Yogi Berra somewhat, "You can see a lot by looking." We know that the interaction of myosin with actin occurs in the region of the whale's chin and lower jaw, with a potential contribution from its nose as well. The 'lower jaw' got hit 6 times in the cold-sensitive mutant hunt: mutants E531Q, P536R, R562H, R562L, E586K and G624D all reside here, as indicated by the cyan colored amino acids in the figure below. Only the lower jaw of the whale is portrayed, along with the 'forehead helix' (magenta). The whale is facing to the right.

So, what have we got to look at here? One particularly intriguing element is the odd 'wrinkle' in the 511-536 helix (yellow, sometimes referred to as ActBind in these pages). Notice the funny loop in its middle? This oddity has the sequence
Gly-Arg-Gln-Pro-Pro-Gly
in Dictyostelium myosin. Given that Gly and Pro residues are unwelcome in a-helices, there is no wonder this region doesn't wind into a helical configuration! So, what are we to think? Firstly, we must wonder whether this is an aberration of the Dicty structure. But if we look at the corresponding part of the chicken structure, look what we behold:

 

 

Further, perusal of the sequences of this region for the whole myosin family suggest this element may be common (sequence alignment viewable at the MRC myosin web site).

To follow the results of our looking, click here to go to Quivering Chin: Inferences

 

What do we intend to do about it?


rove it, of course! As a first step, we have investigated whether the wrinkle is important at all. The sequence shown in bold above was replaced with a single Ala residue. This residue should allow the helix to be made as one continuous element with no internal weakness. The result? Myosin without the Wrinkle are lousy motors. So that's step one--proving that the element is important. Far more difficult is step two: finding out WHY. In order to address this question, we're currently isolating suppressor mutations that restore function to motors without the Wrinkle--and getting them. The preliminary indication is that many of the SAME mutations that fix other mutations in the actin binding region (the cyan fellows shown above) also fix the Wrinkle Deletion! Thus we have forged a link between what the so-called actin binding mutants do and the stage in which the Wrinkle plays a role. We're also taking a good hard look at where these suppressors are in an attempt to deduce the unseen conformation of the chin during actin binding! We are also gearing up to look at the Wrinkle Deletion myosin biochemically, to try to understand exactly what's wrong with such a myosin.

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Bruce Patterson
http://research.biology.arizona.edu/myosin