Bidirectional intracellular transport: utility and mechanism

Abstract

Bidirectional transport of intracellular cargo along microtubule tracks is the subject of intense debate in the motility field. In the present review, we provide an overview of the models describing the possible mechanisms driving intracellular saltatory transport, taking into account current experimental results that may at first seem contradictory. We examine the phenomenon of saltatory motion, in an attempt to interpret the mechanistic debate in terms of the utility of saltatory motion.

Figure 1. Cartoons illustrating different models of bidirectional organelle motility

These models are not necessarily presumed to be mutually exclusive. (A) Opposite-polarity motors undergo tug-of-war and the direction of motion is decided by the recruitment of additional motors (for example in response to signalling). (B) Tug-of-war dictates the direction of movement of the organelle, and the motors engage in a form of cross-talk such that neither motor can walk independently of the other motor. This model attempts to explain the observation that, in many systems, disruption of one motor also abolishes the activity of the opposite-polarity motor. (C) A co-ordinating complex dictates the directionality of motion. Tug-of-war does not occur in this model, and the activity of each motor is dependent upon the signal from the co-ordinating complex. (D) A stochastic mechanism determines the direction of motion, and opposite-polarity motors compete for cargo motility. The net direction of movement is a result of regulated motor activation and/or inactivation by binding partners or post-translational modifications of the motors themselves.