Crystal Structure of the Herpesvirus Nuclear Egress Complex Provides Insights into Inner Nuclear Membrane Remodeling

Abstract

SummaryAlthough nucleo-cytoplasmic transport is typically mediated through nuclear pore complexes, herpesvirus capsids exit the nucleus via a unique vesicular pathway. Together, the conserved herpesvirus proteins pUL31 and pUL34 form the heterodimeric nuclear egress complex (NEC), which, in turn, mediates the formation of tight-fitting membrane vesicles around capsids at the inner nuclear membrane. Here, we present the crystal structure of the pseudorabies virus NEC. The structure revealed that a zinc finger motif in pUL31 and an extensive interaction network between the two proteins stabilize the complex. Comprehensive mutational analyses, characterized both in situ and in vitro, indicated that the interaction network is not redundant but rather complementary. Fitting of the NEC crystal structure into the recently determined cryoEM-derived hexagonal lattice, formed in situ by pUL31 and pUL34, provided details on the molecular basis of NEC coat formation and inner nuclear membrane remodeling.

This bud's for you: mechanisms of cellular nucleocytoplasmic trafficking via nuclear envelope budding

“…A flurry of recent structural studies of the different NECs have provided mechanistic insights as to how pUL34 and pUL31 interact with each other, the immature virion and the lamina to effect NE-budding [34–42]. Together with earlier work, they support a model where tightly-interacting pUL34 and pUL31dimers bind simultaneously to the surface of the nucleocapsid and an unknown partner(s) on the nucleoplasmic side of the INM, to dock the capsid.…”

The absence of p53 during Human Cytomegalovirus infection leads to decreased UL53 expression, disrupting UL50 localization to the inner nuclear membrane, and thereby inhibiting capsid nuclear egress

“…This behavior appears to be a beta herpesvirus phenomenon, although there is some indication of similar behavior in psuedorabies virus (Klupp et al, 2007). There are reports that transiently co-expressed UL50 and UL53 (and their homologues) are capable of deforming the INM (Gonnella et al, 2005; Hagen et al, 2015; Klupp et al, 2007; Lorenz et al, 2015; Milbradt et al, 2012; Passvogel et al, 2015; Reynolds et al, 2001; Sharma et al, 2014; Zeev-Ben-Mordehai et al, 2015), which has begun to be explained through structural analysis of these proteins (as reviewed in (Bigalke and Heldwein, 2015)). Our results (see Figure 4F) confirmed this behavior in the absence of p53 during transient expression.…”

“…In support of our immunofluorescent localization, Dalmonte et al used immuno-EM at late times pi to determine that UL53 was located within large invaginations (IINMs) within the nucleus (Dal Monte et al, 2002). In addition, transient expression of the equivalent PrV proteins (UL31/UL34) produces "speckles", which have been determined to be "vesicle formation in the perinuclear space" (Hagen et al, 2015; Passvogel et al, 2015; Zeev-Ben-Mordehai et al, 2015). We found that the majority of LOX cells formed abundant puncta at the late stages of infection.…”

Nuclear Exodus: Herpesviruses Lead the Way

“…The crystal structures of the NEC from HSV-1, PRV, and HCMV revealed an elongated complex of approximately 80 × 40 × 40 Å (Figure 2) (38–41). UL34 has a globular fold and forms a pedestal.…”