MotorCycle

Myosin's biochemical cycle Intro

The Motor Cycle: Burning ATP, Moving Actin


reasonable view of the events occurring during movement of actin filaments by a myosin motor is depicted. Width of the arrows indicates the "probability" or rate at which a given step occurs, i.e. ATP hydrolysis is rapid and reversible. The tenacity of myosin's interaction with the actin filament is indicated by the "strong binding" etc. labels. Note that the actual events involved in Pi release and stroking are hotly debated, and the series and nature of events in that region of the cycle are highly interpretive! While the motor piece of myosin that we study (also called S1, for subfragment 1) is "just the motor", in the cell, most myosins work as an ordered array of many motors attached to one another by their tail segments (not shown in any of the crystal structures because it has been chopped off). The myosin molecules work as a "team" to move an actin filament, somewhat like a tug-of-war team works to pull a rope--with 2 critical differences: 1) the myosin team pulls the rope past itself, the individual molecules do NOT change their position during the working cycle, and 2) unlike a tug-of-war team, each individual only briefly interacts with the actin filament/rope--much of a given molecules time is spent "flexing"--in the states joined by the very large arrow in the diagram. This corresponds to taking apart the ATP molecule and putting it back together again.




nother view of the cycle is presented below; this linear version is easier to use when discussing how certain mutants affect the cycle. In this version, the state of myosin is shown in the colored ovals and triangles (e.g. the yellow first state is the 'rigor' state in which the myosin head is bereft of nucleotide). The width of each state indicates the amount of time a myosin motor might occupy the state (in the presence of actin for this diagram). The triangles are used to denote very short-lived (perhaps even imaginary!) states. The brown line above the cycle indicates the affinity myosin has for actin at each point; the dotted line indicates weak affinity while the thick line indicates strong affinity. Note that the progressively stronger interaction with actin as the stroke is initiated and carried out is conjectural, tho we do have some evidence in support of the conjecture.

"...a tale told by an idiot, full of sound and fury, signifying nothing" (Shakespeare)


he following is my current view of some of the mechanical events associated with the cycle, and where some of the key mutants fit in. NOTE that it is a synthesis of other's conjectures and my fantasies put together to make a pleasing story: it is a starting point, not a bunch of truths written in stone!
In the Beginning


e start with a myosin molecule that is close to an actin filament and has loaded on its fuel packet (ATP). This step is "easy" and the myosin molecule goes back and forth between the state in which it is "cocked" for its next stroke and has cleaved ATP and the state where it has loaded an ATP and is perhaps still in its "post-stroke" conformation. This is analogous to the tug-of-war guy "flexing", or more accurately, stretching to grab the rope, but only touching it. Looking good, perhaps, but not rendering any help! Myosin at this point has only a "passing interest" in actin--its affinity is low.
Myosin on the Make: looking for Actin


s long as Pi is still in the head of the "whale", the shape of the "lips" and "chin" is in some way inappropriate for more than a passing, infirm interaction with actin. It will "stick" to actin only weakly, and even a small amount of salt will render this interaction largely irrelevant. However, as the myosin casually "investigates" the nearby actin filament, at some point it bumps into it "just right"--an uncommon but "productive" interaction takes place (presumably in its "cocked" conformation). This is the "trigger" for the events that follow. In rapid succession, myosin establishes a firmer contact with actin, Pi (the phosphate that has been cut off from ATP leaving ADP behind) is released.
"Out, out, damn spot!"--release of phosphate (Pi)


his release occurs via the opening of the "Gateway" (residues 238 + 459), giving Pi access to "the Tunnel" (a path out of the "whale" via the "mouth"). It is somewhere in this state of events that we believe the G680V mutation to have its nasty problem--at a point AFTER establishment of fruitful contact with actin, but PRIOR to establishment of full contact, and perhaps AT the point of Gateway opening. I think this mutant cannot "open" the gates, so it grabs actin and gets stuck. It is intriguing to note that many of the mutants that restore function to the G680V mutant, such as those in the Cluster, have a high Basal or "drooling" ATPase--Pi escapes from myosin without getting "permission" from fruitful actin contact.
Getting a move on: the mechanical stroke itself


owever it comes about, release of Pi now "allows" or induces the mechanical motions of the head. One useful way to think about this is to imagine Pi acting as a chock or "safety" preventing movements inside the "whale". Once it is gone, the "whale" can change shape freely. The shape change that it favors, at least when bound to actin, is the "tail-wagging" that is hypothesized to be the movement that translocates the actin filament. The nature of this motion is almost entirely unknown. One interesting conjecture tabled by Huxley and Simmons back in the 60s is that myosin moves actin by a "rolling" contact between the whale's lips and chin and the actin surface. Kim Giese and Jim Spudich have conjectured that the P536R mutation is interrupted in this process. The behavior of the G680V mutant is consistent with the idea that the initial contact with actin is weaker or less extensive than the contacts we observe at the end of the motion.
Letting go (of ADP)--don't be a drag!


nce myosin has "done its thing" and moved the actin filament (or exerted force upon it, should the filament provide great resistance), the ADP is released. This may or may not be a causal relationship: there could be a "timer" in myosin that simply holds the ADP for a certain period after Pi release "assuming" that the stroke will occur. Alternatively, the movement of the head during the stroke may alter the pocket that "holds" ADP, such that at the end of the stroke it is "free to go". Letting the ADP out is a very important step, because as long as it remains in the pocket, ATP cannot enter. A myosin with no nucleotide or with ADP still bound is a dangerous thing--it is like stationary tug-of-war puller who has pulled as far as he can, but now refuses to let go. By so doing, he acts as a "brake" on the others still working. The perfect myosin would let go as soon as it had done its "pull" and was no longer meeting resistance. Unfortunately, myosin, like the rest of us, lives in a real world where everything comes at a cost. If the machine were designed to kick its ADP out too readily, it would periodically do so before its work was done. This in turn could allow an ATP to slip in and force the myosin to "let go" of the actin "rope"--Ooops!
Starting over--loading a new ATP


nce the ADP has wandered away, we are left with a myosin molecule very strongly bound to the actin filament. This state is quite stable, and has been observed extensively with electron microscopy (unfortunately, we are only able to observe the basic shape of the "whale" using this technique--it lacks the resolving power to tell us the position of individual amino acids or secondary structure elements). This myosin quickly finds an ATP (it has a great affinity for ATP, so any ATP molecule that "bumps into it" is caught and kept). The ATP molecule releases the myosin from actin (by changing its shape in some way such that the "fit" between myosin and actin is no longer very good) and we are back at the beginning.

You're in: MotorCycle Places to go:
PREVIOUS

Philosophy

Why mutants
can solve motors

Home

Overview
NEXT

Smoke &
Mirrors

The Origins
of Myosin Truths

One
Bad Motor


Dissecting a
mutant's Defects

Bestiary

Common names
of Parts
NavTable Comments Help!


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