Mapping of sequences in Pseudorabies virus pUL34 that are required for formation and function of the nuclear egress complex.

The nuclear egress complex (NEC) is required for efficient translocation of newly synthesized herpesvirus nucleocapsids from the nucleus to the cytosol. It consists of the type II membrane protein pUL34 which interacts with pUL31 at the inner nuclear membrane (INM). To map regions within pUL34 required for nuclear membrane targeting and pUL31 interaction, we constructed deletion/substitution mutations. Previously, we showed that 50 C-terminal amino acids (aa) of pseudorabies virus (PrV) pUL34, including the transmembrane domain, could be functionally replaced by cellular lamina-associated polypeptide 2β (Lap2β) sequences. In contrast, replacement of the C-terminal 100 aa abrogated complementation but not pUL31 interaction. To further delineate essential sequences within this region, C-terminal pUL34 truncations of 60, 70, 80, 85, and 90 aa fused to Lap2β sequences were generated. While truncations up to 85 aa were functional, deletion of the C-terminal 90 aa abrogated function, which indicates that the important region is located between aa 171 and 176. Amino acids 173 to 175 represent RQR, a motif suggested to mediate INM targeting. Mutagenesis to RQG revealed that the mutant protein exhibited pronounced Golgi localization, but a fraction still reached the INM. Deletion mutations in the N-terminal domain of pUL34 demonstrated that absence of the first 4 aa was tolerated, while removal of 9 or more residues resulted in a nonfunctional protein. In addition, mutation of three conserved cysteines did not abrogate pUL34 function, whereas alteration of a conserved glutamine/tyrosine sequence yielded a nonfunctional protein.