The structure of Factin
Like the monomers themselves, the F-actin polymer (Figure 15.02) is a polar structure with a barbed and a pointed end—terms coined from an early observation of how filaments appeared in electron micrographs when bound to another protein, myosin. Under physiological conditions, filaments grow more rapidly at their barbed than
at their pointed ends, a feature that led to the terms plus and minus end which are used interchangeably with barbed and pointed, respectively.
The F-actin filament can be described both as a right-hand double helix of protofilaments and as a single helix of subunits1 that are rotated relative to one another about the longitudinal axis of the filament. Using the latter description, as we move from the filament minus end toward the plus end, each subunit is rotated by 166° in a clockwise direction relative to the one immediately preceding it. There are 370 sub-units per |im along the length of a filament, which equates to a distance of ~ 27.7Â from the center of one subunit to the next.
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