Abstract
African swine fever virus (ASFV) is a large DNA virus that assembles in viral factories located close to the microtubule organising centre. This study has investigated the mechanisms by which ASFV reaches the cell surface from these perinuclear assembly sites. The first step in this transport is dependent on intact microtubules and involves the motor protein conventional kinesin. This plus-end motor is recruited to virus factories and to individual cytoplasmic virions. Consistent with a role for conventional kinesin during ASFV egress, over-expression of the cargo-binding domain of the kinesin light chain, which acts as a dominant negative protein, severely inhibits movement of viruses to the plasma membrane. Preliminary binding studies show that p73, the main capsid protein, may bind the cargo binding domain of kinesin light chain in vitro. p73 may therefore recruit conventional kinesin to ASFV particles. Once delivered to the periphery of the cell, virions were found associated with microfilaments, possibly using cortical actin fibres as tracks. Once at the plasma membrane, particles induce the formation of long actin projections that morphologically and structurally resemble filopodia. A second generation of particles was capable of moving within these filopodia-like structures in a bidirectional manner. Altogether, these data suggest that ASFV transport from assembly sites to the plasma membrane involves at least four mechanisms: microtubule-based motility, transport along cortical actin filaments, actin polymerisation from the plasma membrane and bi-directional movement along pre-existing projections.