Actin and Myosin

ID #2466

For the synthesis of F actin, my notes say that G actin will collide with the filament, and then if another G actin collides into the G actin that collided with the F actin, it will polymerize the G actin that was jammed between the F actin and G actin. The ATP is then hydrolyzed. Is that correct? I had the idea that this F actin strand would be relatively static in space, but when I was reading the section in the book they talked about treadmilling which make it seem like the strand is continuously moving? I.e. getting shorter on the pointed end and longer on the barbed end. That was for steady state, what happens if you are above the critical concentration? Does the barbed end just grow much faster than the pointed end falls apart?

You have the basic idea, but there are some nuances. When starting an actin filament, you start with the single G actin molecules, but then they begin to stick together into dimers. Then more G actins "stick" to yield trimers and the full F actin begins to form. The process is called nucleation and is depicted on slide 5 of lecture 30. 
As for slide 8, the point of this one is to explain the critical concentrations at the different ends of the F actin molecule. It's not so much one G actin hitting the filament and then the next one locking it into place as it is considering the critical concentration at each end, and that determines whether the G actin will be incorporated. (Stacy explained the critical concentration in more depth a little further up on this page.) 
As for the ATP, each actin contains a nucleotide binding site. The occupancy of this site helps determine the actin's affinity for other actin molecules. If the actin is bound to ATP, then it has a higher affinity and is more likely to be added to a polymerizing end of F actin. But that ATP can be hydrolyzed, lowering the actin's affinity and making it want to leave the F actin. However, this is only possible if the ADP-actin is at one of the ends of the F actin; otherwise, it is blocked in by its neighbors and can't leave. 
The F actin does not have to be static, and the process of polymerizing/depolymerizing will make it appear to "crawl" or treadmill. In terms of being above the critical concentration, it depends both ends of the filament. 
For example: 
- if only the plus end is above the critical concentration, then yes, the plus end would be polymerizing as the minus end is depolymerizing. 
- if the minus end is also above the critical concentration, it will be polymerizing as well. 
- So, if both ends are above the critical concentration, the filament will be growing in both directions. 

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