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Journal of Virology, September 2007, p. 8868-8877, Vol. 81, No. 17
0022-538X/07/$08.00+0     doi:10.1128/JVI.00739-07
Copyright © 2007, American Society for Microbiology. All Rights Reserved.

A Putative Leucine Zipper within the Herpes Simplex Virus Type 1 UL6 Protein Is Required for Portal Ring Formation

University of Connecticut Health Center, 263 Farmington Ave., Room L2037-MC3205, Farmington, Connecticut 06030

Received 5 April 2007/ Accepted 11 June 2007

The herpes simplex virus type 1 UL6 protein forms a 12-subunit ring structure at a unique capsid vertex which functions as a conduit for encapsidation of the viral genome. To characterize UL6 protein domains that are involved in intersubunit interactions and interactions with other capsid proteins, we engineered a set of deletion mutants spanning the entire gene. Three deletion constructs, D-5 (198-295), D-6 (322-416), and D-LZ (409-473, in which a putative leucine zipper was removed), were introduced into the viral genome. All three mutant viruses produced only B capsids, indicating a defect in encapsidation. Western blot analysis showed that the UL6 protein was present in the capsids isolated from two mutants, D-6 and D-LZ. The protein encoded by D-5, on the other hand, was not associated with capsids and was instead localized in the cytoplasm of the infected cells, indicating that this deletion affected the nuclear transport of the portal protein. The UL6 protein from the KOS strain (wild type) and the D-6 mutant were purified from insect cells infected with recombinant baculoviruses and shown to form ring structures as assessed by sucrose gradient centrifugation and electron microscopy. In contrast, the D-LZ mutant protein formed aggregates that sedimented throughout the sucrose gradient as a heterogeneous mixture and did not yield stable ring structures. A mutant (L429E L436E) in which two of the heptad leucines of the putative zipper were replaced with glutamate residues also failed to form stable rings. Our results suggest that the integrity of the leucine zipper region is important for oligomer interactions and stable ring formation, which in turn are required for genome encapsidation.

Published ahead of print on 20 June 2007.