The processes mediating capsid trafficking during human cytomegalovirus nuclear egress

Abstract

Herpesviruses are widespread pathogens that cause or contribute to an array of human diseases. Viral DNA synthesis, capsid assembly, and packaging occur in nucleoplasmic structures called replication compartments (RCs), after which capsids transit to the cytoplasm in a process called nuclear egress. Nuclear egress is orchestrated by a two-subunit complex known as the nuclear egress complex (NEC). While significant progress has been made in understanding how the NEC contributes to steps of nuclear egress at the inner nuclear membrane (INM), little is known about how nascent capsids migrate from RCs to the nuclear periphery. It has been proposed that the nuclear actomyosin system facilitates capsid motility; however, the topic is unresolved and controversial. In this dissertation, we investigated human cytomegalovirus (HCMV) nuclear egress, which is orchestrated by an NEC comprising INM-anchored UL50 and its soluble binding partner, UL53. We uncovered evidence that UL53 associates with capsids and the host motor protein, myosin Va, in infected nuclei. We also found that nuclear actin filaments (F-actin) are induced upon infection and colocalize with myosin Va and capsid protein along the RC and nuclear peripheries and between RCs and the nuclear rim. Antagonism of myosin Va or nuclear F-actin causes defects in the production of infectious virus, capsid accumulation in the cytoplasm, and capsid localization away from RCs. While a population of UL53 colocalizes with capsids and myosin Va in the nucleoplasm, it is not required for capsid localization away from RCs or for the association between capsids and myosin Va. Finally, we preliminarily observed that a population of HCMV capsids undergo directed intranuclear movement in a manner consistent with actomyosin-dependent motility. Collectively, our results suggest that HCMV capsids associate with myosin Va and F-actin in RCs to initiate directed movement to the nuclear periphery, and that UL53 may serve as a passenger protein to coordinate capsid motility with downstream steps of nuclear egress at the INM. While the nuclear cytoskeleton has been implicated in numerous processes, many aspects remain poorly understood. These studies should help pave the way towards a better understanding of herpesvirus nuclear egress and actomyosin-based trafficking in the nucleus.