Distinct Roles of Viral Us3 and Ul13 Protein Kinases in Herpes Virus Simplex Type 1 (Hsv-1) Nuclear Egress

Abstract

Herpesviruses transport nucleocapsids from the nucleus to the cytoplasm by capsid envelopment into the inner nuclear membrane and de-envelopment from the outer nuclear membrane, a process that is coordinated by nuclear egress complex (NEC) proteins, pUL34, and pUL31. Both pUL31 and pUL34 are phosphorylated by the virus-encoded protein kinase, pUS3, and phosphorylation of pUL31 regulates NEC localization at the nuclear rim. pUS3 also controls apoptosis and many other viral and cellular functions in addition to nuclear egress, and the regulation of these various activities in infected cells is not well understood. It has been previously proposed that pUS3 activity is selectively regulated by another viral protein kinase, pUL13 such that its activity in nuclear egress is pUL13-dependent, but apoptosis regulation is not, suggesting that pUL13 might regulate pUS3 activity on specific substrates. We compared HSV-1 UL13 kinase-dead and US3 kinase-dead mutant infections and found that pUL13 kinase activity does not regulate the substrate choice of pUS3 in any defined classes of pUS3 substrates and that pUL13 kinase activity is not important for promoting de-envelopment during nuclear egress. We also find that mutation of all pUL13 phosphorylation motifs in pUS3, individually or in aggregate, does not affect the localization of the NEC, suggesting that pUL13 regulates NEC localization independent of pUS3. Finally, we show that pUL13 co-localizes with pUL31 inside the nucleus in large aggregates, further suggesting a direct effect of pUL13 on the NEC and suggesting a novel mechanism for both UL31 and UL13 in the DNA damage response pathway.

‘Getting Better’—Is It a Feasible Strategy of Broad Pan-Antiherpesviral Drug Targeting by Using the Nuclear Egress-Directed Mechanism?

“…In particular, the regulatory association of cyclin-dependent kinases (CDKs) types 1, 2, and 5 [ 99 , 125 ], as well as the viral ortholog HCMV vCDK/pUL97, was repeatedly demonstrated [ 61 ]. In particular, the role of HCMV pUL97 and related herpesviral kinases, such as EBV BGLF4 as well as HSV-1/-2 pUL13 and pUS3, moved into focus [ 56 , 58 , 61 , 136 , 137 , 138 ]. The distinct functional importance of HCMV pUL97 for the regulated efficiency of viral nuclear egress [ 22 , 139 , 140 , 141 ], and its direct association with viral capsids during the egress process [ 55 ], strongly fostered the validation of this kinase as a target for antiviral therapy.…”