Herpes simplex virus encodes a virion-associated protein which promotes long cellular processes in over-expressing cells

Background: Herpes simplex virus (HSV) possesses a number of accessory genes which are dispensable for replication in cell culture. A previous study showed that the UL21 gene product of HSV type 1 is a virion component that is not necessary for viral replication. The function of the gene product remains unknown. Results: We found that the HSV-1 UL21 gene product, a capsid-associated tegument protein with an apparent molecular mass of 62 kDa, promotes the outgrowth of long cellular processes when it is over-expressed in non-neural cells. The UL21 protein co-localizes and physically associates with microtubules in the long processes. Analysis using mutant proteins implicates a proline-rich region in promotion of the processes. Conclusions: The results suggest that the UL21 protein, like tau and other MAPs, promotes the process by directly or indirectly interacting with microtubules and facilitates the intracellular transport of the virus.