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Figure 1. Basic functions for plus and minus end-directed myosins in cells. Myosin drives oriented transportation of cargo (A) and sliding of actin filaments (B). Myosin head (black balls) and tail (black stick) domains, actin filaments (red, blue lines), vesicle (circle), plasma membrane (curved and straight lines). Both indicated types of actin organization (A and B) exist in nonmuscle motile cells. A, Myosin pulls cargo on a uniform filament polarity actin network. Plus end- (upward arrow) and minus end- (downward arrows) directed myosins transport cargo in opposite directions. Myosins I and V are known plus end-directed transport motors. Myosin VI, the minus end-directed actin motor, also has transport motor activity in cells, but oriented movement of cargo remains to be demonstrated (see text). B, Myosin slides actin filaments in an opposite filament polarity (antiparallel) actin network. A plus end-directed myosin (top) pulls actin filaments together (thick arrows move together), generating pulling or contraction force in the cell. A minus end-directed myosin (bottom) pushes filaments apart (thick arrows move apart), generating pushing or expansion force in the cell. Contraction force generated by myosin II has been documented in many systems. However, expansion force resulting from minus end-directed myosins is, at this point, hypothetical.
- Two distinct types of actin organization that exist in cells, uniform and opposite filament polarity actin networks (see below for descriptions), allow two extreme types of myosin function, respectively (Fig 1): oriented transportation of cargo (Fig 1 A) and sliding of actin filaments (Fig 1 B).
- To drive cargo transport, myosin (Fig 1 A, single black ball and stick) moves (Fig 1 A, arrows) attached cargo (Fig 1 A, circles, curved line) over the surface of a uniform filament polarity actin network (Fig 1 A, red lines).
- In this actin organization, all actin filament plus ends face towards the cell surface (Fig 1 A, as indicated).
- This allows oriented transportation of cargo into or out of the cell (e.g., Fig 1 A, compare upward and downward arrows).
- Thus, a plus end-directed myosin will transport attached cargo outwards towards the cell surface, such as a vesicle on the exocytic pathway (Fig 1 A, upward arrow).
- Conversely, a minus end-directed myosin will transport attached cargo inwards, away from the cell surface, such as a vesicle on the endocytic pathway or a region of the plasma membrane that is being retracted or tethered by the myosin (Fig 1 A, downward arrows).
- To drive actin filament sliding, myosin oligomers (Fig 1 B, two black balls and stick) sit between repeating units of actin filament subbundles (e.g., two subbundles are represented by red and blue lines in Fig 1 B) in an opposite filament polarity actin network, and move subbundles relative to one another (e.g., Fig 1 B top, compare thick arrows).
- In this actin organization, actin filament plus ends from adjacent subbundles face opposite directions (Fig 1 B, compare red and blue lines) and myosin sits between overlapping/interdigitating filament minus ends from adjacent subbundles (Fig 1 B, as indicated).
- Also, net filament plus ends are anchored to the plasma membrane (e.g., Fig 1 B, junction of vertical and horizontal lines) via adhesion complexes or other connections.
- Thus, filament sliding driven by a plus end-directed myosin in an opposite filament polarity actin network exerts net pulling force or cell contraction force (Fig 1 B, upper panel, arrows move together).
- Conversely a minus end-directed myosin in the same actin network exerts net pushing force or cell expansion force (Fig 1 B, bottom, arrows move apart).