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Journal of Virology, October 2001, p. 8927-8936, Vol. 75, No. 19
0022-538X/01/$04.00+0   DOI: 10.1128/JVI.75.19.8927-8936.2001
Copyright © 2001, American Society for Microbiology. All rights reserved.

Pseudorabies Virus UL37 Gene Product Is Involved in Secondary Envelopment

Barbara G. Klupp,1 Harald Granzow,2 Egbert Mundt,1 and Thomas C. Mettenleiter1,*

Institutes of Molecular Biology1 and Infectology,2 Friedrich-Loeffler-Institutes, Federal Research Centre for Virus Diseases of Animals, D-17498 Insel Riems, Germany

Received 10 May 2001/Accepted 25 June 2001

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 membrane releases nucleocapsids into the cytoplasm, which then gain their final envelope by budding into trans-Golgi vesicles. It has been shown that the UL34 gene product is required for primary envelopment of the alphaherpesvirus pseudorabies virus (PrV) (B. G. Klupp, H. Granzow, and T. C. Mettenleiter, J. Virol. 74:10063-10073, 2000). For secondary envelopment, several virus-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. Mettenleiter, J. Virol. 73:5364-5372, 1999). We show here that the product of the UL37 gene of PrV, which is a constituent of mature virions, is involved in secondary envelopment. Replication of a UL37 deletion mutant, PrV-Delta UL37, was impaired in normal cells; this defect could be complemented on cells stably expressing UL37. Ultrastructural analysis demonstrated that intranuclear capsid maturation and budding of capsids into and release from the perinuclear space were unimpaired. However, secondary envelopment was drastically reduced. Instead, apparently DNA-filled capsids accumulated in the cytoplasm in large aggregates similar to those observed in the absence of glycoproteins E/I and M but lacking the surrounding electron-dense tegument material. Although displaying an ordered structure, capsids did not contact each other directly. We postulate that the UL37 protein is necessary for correct addition of other tegument proteins, which are required for secondary envelopment. In the absence of the UL37 protein, capsids interact with each other through unknown components but do not acquire the electron-dense tegument which is normally found around wild-type capsids during and after secondary envelopment. Thus, apposition of the UL37 protein to cytoplasmic capsids may be crucial for the addition of other tegument proteins, which in turn are able to interact with viral glycoproteins to mediate secondary envelopment.

* Corresponding author. Mailing address: Institute of Molecular Biology, Friedrich-Loeffler-Institutes, Federal Research Centre for Virus Diseases of Animals, Boddenblick 5A, D-17498 Insel Riems, Germany. Phone: 49-38351-7250. Fax: 49-38351-7151. E-mail: mettenleiter{at}rie.bfav.de.

Journal of Virology, October 2001, p. 8927-8936, Vol. 75, No. 19
0022-538X/01/$04.00+0   DOI: 10.1128/JVI.75.19.8927-8936.2001
Copyright © 2001, American Society for Microbiology. All rights reserved.


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Copyright © 2001 by the American Society for Microbiology. All rights reserved.