Stochastic description of pilus retraction dynamics

Motility of certain gram-negative bacteria is mediated by retraction of type IV pili surface filaments, which are essential for infectivity. Type IV pili are helical filaments with 4 nm periodicity and 5 subunits per turn. The retraction is powered by a strong molecular motor protein, PilT, producing very high forces in excess of 100 pN[1]. One possible explanation for the high forces are that several ATP are hydrolyzed to retract each subunit.\\ We consider a widely used class of discrete hopping models, which has been used to describe well-known motor proteins such as kinesin[2] and myosin[3]. The model describes recent experimental measurements[1] on \emph{Neisseria gonorrhoeae} well, and makes several interesting predictions for the randomness of the retraction dynamics.\medskip\\ 1. Maier et al, PNAS 101:10961 (2004)\\ 2. M. E. Fisher and A. B. Kolomeisky, PNAS 98:7748 (2001)\\ 3. A. B. Kolomeisky and M. E. Fisher, Biophys. J. 84:1650 (2003)