nother bit of interesting data comes from looking at several kinds of experiments performed on in this part of the protein. Crosslinking, phylogenetic comparison, and genetic suppression experiments all hint at juxtapositions of the lower jaw and 'Adam's apple' that simply aren't apparent in the current structure. A lovely figure showing these relationships:

Here, the whale faces to the LEFT. Positions of cold-sensitive mutants are shown in cyan. Orange balls with green stripes indicate positions of residues that can mutate to suppress defects arising from a mutation at the residue shown as green with orange stripes. The zebra-striped pair are residues that co-vary phylogenetically. The blue residues with yellow stripes can be crosslinked, as can the magenta residues with yellow stripes (More on crosslinking in the next section). In general, note that there seems to be some sort of crosstalk between stuff on the left and stuff on the right (e.g. the zebra striped residues which appear to have a relationship over evolutionary time periods are NOT close togethe in space in this structure).

 

Xlinks: Catching myosin with its hand in the cookie jar

One technique that has been used to try to glimpse short-lived structures that occur during the myosin cycle is crosslinking (abbreviated as xlinking herein). In this technique, amino acid residues (most commonly Cysteines) are chemically activated. Should they come very close to appropriate other residues (again commonly other cysteines) they chemically bond with them. These permanent bonds thus represent a juxtaposition that occurred at some time even though it may be transient and not part of the 'resting' structure. There are two crosslinks that impinge on our story: one between position 533 and position 680 and one between position 513 and position 691 (actually, the xlinks were found in rabbit myosin, but I have 'translated' their positions to the Dicty equivalents. One more thing you need to know: The technique used to chemically activate the cysteines often involves putting on a 'linker' molecule that in effect extends the length of the amino acid. In the figure below, spheres have been used to indicate the area that the relevant activated cysteine could 'reach' to make contact with another cysteine.

In the image, the whale is facing left. Notice that all points on the cyan 'beachball' fall far short of the cyan residue in the yellow helix--yet at some point in the myosin cycle, a linkage can be made. Similarly, the lime beachball does not encompass the lime residue (center right). Again, the linkage can occur. The key point here is that even with the extensions, the observed crosslinks cannot be explained within the context of the structure shown. Might they be hinting at one of myosin's other structures? Notice that the same general form of movement would be required to allow BOTH xlinks--the yellow helix would need to move leftward and into the page.

 

So, What's the Hypothesis?

imply put, that there's "a whole lotta shakin' goin' on" in this part of myosin, and if so, it's happening during the actin binding states! More eloquently put, I suggest that there is a significant re-arrangement of structures in this area as a part of and prerequisite to strong actin binding. The 'Wrinkle' is a key element, acting perhaps as a hinge, perhaps simply as an instability element in the 511-536 helix, allowing it to 'come undone' as the chin re-arranges.

 

 

What do we intend to do about it? Experimental approaches and progress can be found HERE.