SUMMARY Microtubule transport of herpesvirus capsids from the cell periphery to the nucleus is imperative for viral replication and, in the case of many alphaherpesviruses, transmission into the nervous system. Using the neuroinvasive herpesvirus, pseudorabies virus (PRV), we show that the viral protein 1/2 (VP1/2) tegument protein associates with the dynein/dynactin microtubule motor complex and promotes retrograde microtubule transport of PRV capsids. Functional activation of VP1/2 requires binding to the capsid protein pUL25 or removal of the capsid-binding domain. A proline-rich sequence within VP1/2 is required for the efficient interaction with the dynein/dynactin microtubule motor complex as well as for PRV virulence and retrograde axon transport in vivo. Additionally, in the absence of infection, functionally active VP1/2 is sufficient to move large surrogate cargoes via the dynein/dynactin microtubule motor complex. Thus, VP1/2 tethers PRV capsids to dynein/dynactin to enhance microtubule transport, neuroinvasion, and pathogenesis.

The herpesvirus virion is a multilayered structure consisting of a DNA-filled capsid, tegument, and envelope. Detailed reconstructions of the capsid are possible based on its icosahedral symmetry, but the surrounding tegument and envelope layers lack regular architecture. To circumvent limitations of symmetry-based ultrastructural reconstruction methods, a fluorescence approach was developed using single-particle imaging combined with displacement measurements at nanoscale resolution. An analysis of 11 tegument and envelope proteins defined the composition and plasticity of symmetric and asymmetric elements of the virion architecture. The resulting virion protein map ascribes molecular composition to density profiles previously acquired by traditional ultrastructural methods, and provides a way forward to examine the dynamics of the virion architecture during infection.pseudorabies | virus | heterogeneity | asymmetry | point-spread function H erpesviruses are responsible for a broad range of diseases in humans and other animals. The herpesvirus virion consists of four components: (i) the linear dsDNA genome, (ii) a 125-nm diameter T = 16 icosahedral capsid, (iii) a tegument consisting of more than 20 proteins that surround the capsid, and (iv) a lipid bilayer envelope studded with viral glycoproteins. The fully assembled particle is ∼200-250 nm in diameter and is referred to as the heavy particle (H-particle) (1). The capsid has been solved to 8.5 Å resolution and a fraction of the tegument that is symmetrically bound to the capsid surface has been solved to 20 Å resolution by cryo-electron microscopy (cryo-EM) reconstruction (2-4). The detailed resolution afforded by cryo-EM results from the averaging of many particles that are aligned in silico, based on icosahedral symmetry. However, such studies provide an incomplete picture of herpesvirus virions because unlike some smaller enveloped viruses that project icosahedral symmetry into the envelope proteins, the herpesvirus envelope, and the majority of the tegument mass lack radial symmetry and are not "seen" by cryo-EM (5-7). Our understanding of these variable structural layers predominantly comes from single-particle imaging methods that do not use symmetry-based averaging. In particular, cryo-electron tomography (cryo-ET) has provided insight into the general structure of the outer virion layers, but has not yielded sufficient detail to resolve the organization of the constituent protein components (8).Extracellular herpes virions are metastable structures that are triggered by interactions between virion membrane surface proteins and corresponding receptors on the cell, culminating in membrane fusion (9). Although tegument proteins are critical for postfusion steps in nuclear delivery (10-14), 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). A de...

This study provides novel mechanistic insights into how Syt-1 and Syt-7 C2B domains regulate the later stages of exocytosis. We show that small structural differences within this region exert significant effects on fusion pore properties. We propose that these effects may arise from differences in C2B domain affinity for the plasma membrane.

A lumenal secretory granule protein can slow fusion pore dilation and thus its own discharge. Bohannon et al. demonstrate another outcome: the creation of a nanoscale chemical reaction chamber for granule contents in which the pH is suddenly neutralized upon fusion.

In order to resolve the location and activity of submicroscopic viruses in living cells, viral proteins are often fused to fluorescent proteins (FPs) and visualized by microscopy. In this study, we describe the fusion of FPs to three proteins of pseudorabies virus (PRV) that allowed imaging of capsids in living cells. Included in this study are the first recombinant PRV strains expressing FP-pUL25 fusions based on a design applied to herpes simplex virus type 1 by Homa and colleagues. The properties of each reporter virus were compared in both in vitro and in vivo infection models. PRV strains expressing FP-pUL25 and FP-pUL36 preserved wild-type properties better than traditional FP-pUL35 isolates in assays of plaque size and virulence in mice. The utility of these strains in studies of axon transport, nuclear dynamics and viral particle composition are documented.