Role of ATP/ADP in Microfilament Assembly
The actin monomers are color coded as indicated. Only monomers with bound ATP assemble into the microfilament. Click here to see an x-ray crystal structure of a beta actin molecule with a bound ATP. In the X-ray crystal image, the actin molecule is shown in blue and the single ATP molecule is yellow. The much smaller protein shown in green (profilin) is also shown in this crystal structure. Shortly after actin monomers assemble into filaments, the ATP is hydrolyzed to ADP. The ADP remains bound to the filamentous actin. Thus, on the + end of the microfilament there is a cap of ATP containing monomers (shown in yellow in the animation), but most of the microfilament is composed of monomers binding ADP (shown in blue in the animation). As new ATP containing monomers are added, the oldest ATP containing monomers are hydrolyzing their ATP to ADP. Thus, even though the microfilament is elongating on the + end, the size of the cap stays the same. On the - end of the microfilament, it is the ADP containing monomers which dissassociate from the filament. In this animation, the rate of association is equal to the rate of dissassociation and so there is no net elongation or shortening of the microfilament. All of the actin monomers assemble in a defined orientation, so that even in the absence of the ATP/ADP differences, the microfilament would still have a molecularly defined directionality or polarity. Click here to see an illustration of this polarity. Other molecules in the cell (such as myosin) can read this polarity and can thus interact with microfilaments in directionally specific ways.
Learn about treadmilling microfilaments
Back to the Directory of Animations
Back to Berg's Homepage