  Systems with two species of active molecular motors moving on (cytoskeletal)
filaments into opposite directions are studied theoretically using driven
lattice gas models. The motors can unbind from and rebind to the filaments. Two
motors are more likely to bind on adjacent filament sites if they belong to the
same species. These systems exhibit (i) Continuous phase transitions towards
states with spontaneously broken symmetry, where one motor species is largely
excluded from the filament, (ii) Hysteresis of the total current upon varying
the relative concentrations of the two motor species, and (iii) Coexistence of
traffic lanes with opposite directionality in multi-filament systems. These
theoretical predictions should be experimentally accessible.
