ecause protein structures relate only poorly to everyday experience, and since only the Few love myosin so well as to know what amino acid #s constitute which structures, we and others have evolved a host of helpful designations for the diverse structural elements that constitute the myosin motor. Below is a collection of terms that I find to be helpful in remembering what's what as well as generating a mental image of the structures involved. Please be advised that these metaphors are extremely loose, and the behavior of the namesake cannot be used to gain insight into the behavior of the protein part! Also, it's worth pointing out that none of these terms bear any sanction from the field, and probably won't help you communicate with anybody but me or my lab. For those of you more intimate with the structure, I will include the amino acid #s for the components of the described structures (at least in the mythical final version of this page).
t's inescapable: everybody who looks at the structures, regardless of scientific acumen, sees the whale in them. The illustration shows the structure Rayment's group deduced for chicken myosin; the Dicty structure is an excellent match to it. The analogy can be forced to go much, much deeper: the ATP molecule enters through the "blowhole", important hypotheses about the nature of myosin's movements involve movements of the upper and lower "jaws" relative to one another, the phosphate molecule exits through the side of the "mouth", and the total (conjectured) movement achieved can be thought of as the wagging of the tail. The tail as shown actually constitutes 2 proteins: parts of the main piece ('heavy chain') with the 'essential' light chain (pinkish) wrapped around the tail.
his is the site that holds the energy packet or ATP molecule. You can't tell from this view, but there is an actual "cave" inside the 3-D structure. The ATP enters this cave, and is held here until the end of the cycle.
his distinction arises from the observation of a cleft or canyon running perpendicular to the blowhole. In the image above, the gap separating them is indicated as the "mouth" Rayment's group has proposed that a closing of this cleft accompanies and drives the stroking motion ("tail-wagging") of the molecule; my current bias is that the lower jaw slides along the long axis of the whale, rendering the "whale" more or less buck-toothed, as it were.
hile the upper and lower jaws are largely encoded by distinct parts of the protein, one very interesting projection comes up from the lower jaw to the upper. It is difficult to contemplate a significant motion of the jaws relative to one another that would not perturb the tusk; thus it is interesting to conjecture that this element plays a role in "braking" or facilitating such motions. Erik Misner has identified mutations that restore function to actin binding site mutants and lie in this element. In the chicken structure, several amino acids from this element are "invisible" since they did not adopt uniform shapes during the crystallization process. The missing residues are known to be involved in making early contacts with actin. Such contacts could clearly influence the shape or behavior of the tusk with regard to the upper jaw.
hese are not necessarily structurally distinct parts of the whale, but are important to point out since these regions are known to contact the actin molecule. These are the leftmost and front-bottom parts of the structure. Thus the whale "kisses" actin more than it bites it, but it's a pretty firm kiss.
crucial part of the myosin cycle involves the exodus of the phosphate molecule that has been liberated from the ATP molecule (adenosine TRIphosphate is cut into adenosine DIphospate and a free phosphate). Yount et al. conjectured that the freed phosphate molecule must exit by a path other than the blowhole, since the phosphate is at the bottom of the blowhole and the ADP still blocks the rest of it. They identified a potential exit route that leads to the "mouth" or 'cleft'. In the image, you can (hopefully) see the shaded cyan BeF molecule, and perhaps make out the orange and purple guardians (R238 and E459, respectively). The tunnel exits over the Camshaft (blue). You are looking obliquely at the front right side of the "whale".
his is a helix that runs along the 'cleft' of the mouth. Note that in this view we're back to Dicty structure, which doesn't include the "tail" section and are somewhat disorganized in the region where it was cut off. Also, the "whale" is "facing" the right side; it has been rotated 180 degrees. I have "cheated" to highlight some points: only the camshaft is shown in "ribbon" representation, and only those amino acids that we have recovered in mutant hunts are shown in their 3-D glory and colored magenta. Webster's defines a cam as "an eccentric wheel mounted on a rotating shaft and used to produce variable or reciprocating motion in another...contacted part." In other words, the shape of the structure and its rotational position determines what "pressures" it is exerting on surrounding parts of the molecule. For those unfamiliar with the role of the camshaft in your average engine, may I suggest Automotive 101's site? Anyway, I believe this helix to be critical because it has turned up (no pun intended) time and again in our search for genetic changes that alter the behavior or the motor. I believe it is a major player in the whale's "state of mind" and thus what shape the whale adopts. Another analogy (suggested and favored by Kim Giese) would be a key--when the bumpy parts of the key slide into a certain position, surrounding parts of the lock move to accommodate it, and if everything is right, the lock opens--you'll see the relevance of this below.
|The Guardians of the Gate
r, for those of you who favor Ghostbusters, the Gatekeeper and the Keymaster... Two residues in myosin, R238 and E459 form a 'salt bridge', an interaction between the positive charges (derived from nitrogen and thus depicted as blue balls) on the R (arginine) residue and the negative ones (derived from oxygen and thus red balls) on the E (glutamic acid). In the vanadate structure (purple), they are close together (fitting, since vanadate [melon] mimics an ATP molecule undergoing separation of ADP and phosphate), while they have a more relaxed configuration in the BeF structure (blue)(BeF [cyan] mimics the behavior of a phosphate in ATP itself--i.e. the phosphate would be firmly attached to the ADP and couldn't go anywhere anyway). Note that residue E459 is glued to the Camshaft, and rotation of the Camshaft moves residue E459 toward or away from R238. The striking similarity to a bucking bronco is of unclear significance.
his group of residues turned up primarily in the hunt for suppressors of the G680V mutation. In the image, they're shown as green. Each green residue, when altered (almost invariably to a larger, hydrophobic residue, and most often a Leu->Phe, e.g. L175F, L453F, T189I...) can restore function to a motor bearing the G680V defect. Residues in the same stretch of amino acids (175-193) that were not hit are shown in blue, while the Camshaft is depicted as red. The L478 position is shown in magenta. This is the "contact point" where the cluster comes closest to the Camshaft. The "whale" would be facing to your right. Biochemically, these alterations are united in that all but I177M markedly increase the Basal ATPase of myosin, and they usually give poor actin activation.
his is an odd-looking aberration in the lower jaw of the whale. It occurs in the middle of an a-helix that runs from amino acids 511-536 (yellow) in the Dictyostelium sequence. The Wrinkle itself emerges from the sequence 519-GRWPPG524. It's location relative to putative actin binding sites associates it with these functions. Replacing the Wrinkle with a single alanine residue renders myosin defective; we are characterizing suppressors of this defect, and current findings indicate that these overlap with suppressors of mutations at positions 531 and 536 (deemed actin binding site mutants)!