  We study the effect of a coupling between two motor domains in
highly-processive motor protein complexes. A simple stochastic discrete model,
in which the two parts of the protein molecule interact through some energy
potential, is presented. The exact analytical solutions for the dynamic
properties of the combined motor species, such as the velocity and dispersion,
are derived in terms of the properties of free individual motor domains and the
interaction potential. It is shown that the coupling between the motor domains
can create a more efficient motor protein that can move faster than individual
particles. The results are applied to analyze the motion of helicase RecBCD
molecules.
