Pseudorabies virus UL37 gene product is involved in secondary envelopment

Herpesvirus envelopment is a two-step process which includes acquisition of a primary envelope resulting from budding of intranuclear capsids through the inner nuclear membrane. Fusion with the outer leaflet of the nuclear me mbrane releases nucleocapsids into the cytoplasm, which then gain their fin al envelope by budding into trans-Golgi vesicles. It has been shown that th e UL34 gene product is required for primary envelopment of the alphaherpesv irus pseudorabies virus (PrV) (B. G. Klupp, H. Granzow, and T. C. Mettenlei ter, J. Virol. 74:10063-10073, 2000). For secondary envelopment, several vi rus-encoded PrV proteins are necessary, including glycoproteins E, I, and M (A. R. Brack, J. M. Dijkstra, H. Granzow, B. G. Klupp, and T. C. Mettenlei ter, J. Virol. 73:5364-5372, 1999). We show here that the product of the UL 37 gene of PrV, which is a constituent of mature virions, is involved in se condary envelopment. Replication of a UL37 deletion mutant, PrV-Delta UL37, was impaired in normal cells; this defect could be complemented on cells s tably expressing UL37. Ultrastructural analysis demonstrated that intranucl ear capsid maturation and budding of capsids into and release from the peri nuclear space were unimpaired. However, secondary envelopment was drastical ly reduced. Instead, apparently DNA-filled capsids accumulated in the cytop lasm in large aggregates similar to those observed in the absence of glycop roteins E/I and M but lacking the surrounding electron-dense tegument mater ial. Although displaying an ordered structure, capsids did not contact each other directly. We postulate that the UL37 protein is necessary for correc t addition of other tegument proteins, which are required for secondary env elopment. In the absence of the UL37 protein, capsids interact with each ot her through unknown components but do not acquire the electron-dense tegume nt which is normally found around wild-type capsids during and after second ary envelopment. Thus, apposition of the UL37 protein to cytoplasmic capsid s may be crucial for the addition of other tegument proteins, which in turn are able to interact with viral glycoproteins to mediate secondary envelop ment.