  Nuclear pore complexes are constantly confronted by large fluxes of
macromolecules and macromolecular complexes that need to get into and out of
the nucleus. Such bi-directional traffic occurring in a narrow channel can
easily lead to jamming. How then is passage between the nucleus and cytoplasm
maintained under the varying conditions that arise during the lifetime of the
cell? Here, we address this question using computer simulations in which the
behaviour of the ensemble of transporting cargoes is analyzed under different
conditions. We suggest that traffic can exist in two distinct modes, depending
on concentration of cargoes and dissociation rates of the transport
receptor-cargo complexes from the pores. In one mode, which prevails when
dissociation is quick and cargo concentration is low, transport in either
direction proceeds uninterrupted by the other direction. The result is that
overall-traffic-direction fluctuates rapidly and unsystematically between
import and export. Remarkably, when cargo concentrations are high and
dissociation is slow, another mode takes over in which traffic proceeds in one
direction for a certain extent of time, after which it flips direction for
another period. The switch between this, more regulated, mode of transport and
the other, quickly fluctuating state, does not require an active gating
mechanism but rather occurs spontaneously through the dynamics of the
transported particles themselves. The determining factor for the behaviour of
traffic is found to be the exit rate from the pore channel, which is directly
related to the activity of the Ran system that controls the loading and release
of cargo in the appropriate cellular compartment.
