  Molecular motors of the kinesin-1 family move in a directed and processive
fashion along microtubules (MTs). It is generally accepted that steric
hindrance of motors leads to crowding effects; however, little is known about
the specific interactions involved. We employ an agent-based lattice gas model
to study the impact of interactions which enhance the detachment of motors from
crowded filaments on their collective dynamics. The predictions of our model
quantitatively agree with the experimentally observed concentration dependence
of key motor characteristics including their run length, dwell time, velocity,
and landing rate. From the anomalous stepping statistics of individual motors
which exhibit relatively long pauses we infer that kinesin-1 motors sometimes
lapse into an inactive state. Hereby, the formation of traffic jams amplifies
the impact of single inactive motors and leads to a crowding dependence of the
frequencies and durations of the resulting periods of no or slow motion. We
interpret these findings and conclude that kinesin-1 spends a significant
fraction of its stepping cycle in a weakly bound state in which only one of its
heads is bound to the MT.
