Figure 1. (a) The modular nature of molecular motors. Microtubule molecular motors (kinesins and dyneins) and (b) actin molecular motors (myosins). Although for all three families of motors the overall topology can vary widely, within a family the track binding domains (red=myosin, purple=kinesin, yellow=dynein) show very high structural and functional homology. The motors are modular: each typically consists of a stalk (containing flexible hinges) and (usually) two identical ATPase head domains, which bind to the track and are connected to the stalk via a neck domain. In kinesins, the structure and mode of attachment of the neck domain to the head determines the progress direction, while the presence or absence of a flexible neck linker domain appears to govern processivity (walking ability, see text). In myosins, the structure and mode of attachment of a semi-stiff lever arm attached to the head(s) determines velocity and progress direction.

Figure 2. Interphase intracellular transport. Microtubules (blue) originate from an organizing centre close to the nucleus, and extend towards the cortex. Actin filaments are enriched at the cortex. Transport is mediated by kinesins (purple), myosins (red) and dyneins (yellow). Some examples are shown. Exocytotic vesicles bud from the trans-Golgi and are transported along MTs to the cortex by kinesins, where they are handed over to myosins for trafficking within the actin-dense subcortical region, followed by fusion (itself involving myosins) and exocytosis. Myosins are also involved in forming endocytotic and pinocytotic vesicles, which are then transported inwards along microtubules by dyneins. Bidirectional trafficking of protein rafts also occurs, for example within flagella.

Figure 3. Melanosome transport in melanocytes. The melanocyte system is the best understood example of regulated vesicle transport. The dynein molecular motor interacts with cargos via the dynactin complex. It is possible that dynein localization is controlled by the localization of the partner proteins dynamitin and P150^Glued. Dynein drives aggregation, by powering MT minus end-directed movement of melanosomes. Dispersion is due to plus end-directed movement, driven by kinesin-2, and to the action of myosin V, which works actively to tether melanosomes to the actin filament cytoskeleton, engaging in a kind of tug o’ war with the microtubule motors. The final distribution of melanosomes within the cell depends on the cyclic AMP concentration, which determines the outcome of this tug o’ war. The KAP subunit of kinesin-2 binds directly to P150^Glued. Myosin V recruitment to the melanosome is controlled by Rab27a. The GTP form of Rab27a recruits a protein called melanophilin that in turn binds myosin V.