Translocation of Incoming Pseudorabies Virus Capsids to the Cell Nucleus Is Delayed in the Absence of Tegument Protein pUL37

Abstract

After fusion of the envelope of herpesvirus particles with the host cell plasma membrane, incoming nucleocapsids are transported to nuclear pores. Inner tegument proteins pUL36, pUL37, and pUS3 remain attached to the nucleocapsid after entry and therefore might mediate interactions between the nucleocapsid and cellular microtubule-associated motor proteins during transport. To assay for the role of pUL37 in this process, we constructed a pUL37-deleted pseudorabies virus mutant, PrV-⌬UL37/UL35GFP, which expresses a fusion protein of green fluorescent protein (GFP) and the nonessential small capsid protein pUL35, resulting in the formation of fluorescently labeled capsids. Confocal laser-scanning microscopy of rabbit kidney cells infected with PrV-⌬UL37/UL35GFP revealed that, whereas penetration was not affected in the absence of pUL37, nuclear translocation of incoming particles was delayed by approximately 1 h compared to PrV-UL35GFP, but not abolished. In contrast, phenotypically complemented pUL37-containing virions of PrV-⌬UL37/UL35GFP exhibited wild type-like entry kinetics. Thus, the presence of pUL37 is required for rapid nuclear translocation of incoming nucleocapsids.The herpesvirus replication cycle starts with attachment of extracellular virions to cellular receptors, which is followed by fusion of the virion envelope either with the cellular plasma membrane (20) or, after endocytosis, with the endosomal membrane (49), resulting in release of the nucleocapsid into the cytosol. During this process, most proteins of the tegument, which in the herpes virion resides between the envelope and nucleocapsid, are released from the incoming particle. Several tegument proteins prime the cell for virus infection by shutting off cellular protein synthesis (virion host shut-off factor, pUL41) (29) or, after translocation into the nucleus, boost viral transcription (␣-transinducing factor, pUL48) (2). Other tegument proteins, however, remain bound to the incoming nucleocapsid during its transit to the nuclear pore, where the viral genome is released into the nucleus to start viral gene expression. Thus, efficient transport of nucleocapsids to the nuclear pore is crucial for herpesvirus infectivity.For efficient nuclear targeting of incoming nucleocapsids, herpesviruses recruit cellular motor proteins to move along microtubules (MT) (9,11,17,43,45,48). This process is particularly relevant to overcome even long distances, such as between neuronal axon termini and the nucleated cell bodies. MT are polar cytoskeletal filaments with a fast-growing plus end, extended toward the cell periphery, and a less dynamic minus end, attached to the MT organizing center (38). Minus end-directed transport catalyzed by the huge dynein-dynactin motor complex (44, 53) is used by a variety of viruses (9,15,30,35,40,41,45,48,50). Studies of epithelial and neuronal cells revealed that incoming herpes simplex virus type 1 (HSV-1) capsids colocalize with cytoplasmic dynein and dynactin and that capsid transport to the nucleus is inhibited by...

Phenotypic similarities and differences between UL37-deleted pseudorabies virus and herpes simplex virus type 1

“…PrV pUL37 has also been demonstrated to be non-essential for entry, although the transport of incoming capsids via the cellular microtubule system to the nuclear pore is delayed in the absence of pUL37 (Krautwald et al, 2009). Taken together, these results imply that pUL37 plays a more important role for virion formation of HSV-1 than of PrV, despite the similarity in ultrastructural phenotypes as observed in the present study.…”

Differential protein partitioning within the herpesvirus tegument and envelope underlies a complex and variable virion architecture

“…Although tegument proteins are critical for postfusion steps in nuclear delivery (10)(11)(12)(13)(14), their perceived role before cell entry has been as rigid structural elements that bridge the capsid shell to the tails of envelope membrane proteins (15,16). However, recent evidence indicates that the tegument may reorganize before membrane fusion (17)(18)(19).…”

The Unusual Fold of Herpes Simplex Virus 1 UL21, a Multifunctional Tegument Protein

“…A unique feature of all herpesviruses is a multiprotein tegument layer between the capsid and the envelope. Besides being necessary for viral assembly, tegument proteins also play critical roles at early stages of the viral replication cycle, in which some are released to regulate expression of viral (3) or cellular genes (4) while others remain bound to the capsid and mediate its trafficking (5)(6)(7). The involvement of tegument proteins at multiple stages during viral replication makes them attractive antiviral targets, yet few have been characterized in detail.…”