Pseudorabies virus glycoprotein K requires the UL20 gene product for processing

Glycoprotein EI. (gK) of pseudorabies virus (PrV) has recently been identif ied as a virion component which is dispensable for viral entry but required for direct cell-to-cell spread. Electron microscopic data suggested a poss ible function of gK in virus egress by preventing immediate fusion of relea sed virus particles with the plasma membrane (B. G. Klupp, J. Baumeister, P . Dietz, H. Granzow, and T. C. Mettenleiter, J. Virol. 72:1939-1958, 1998). For more detailed analysis, a PrV mutant with a deletion of the UL53 (gK) open reading frame (ORF) from codons 48 to 275 was constructed, and the pro tein was analyzed with two monoclonal antibodies directed against PrV gK. T he salient findings of this report are as follows. (i) From the PrV UL53 OR F, a functional gK is translated only from the first ire-frame methionine. Front the second in-frame methionine, a nonfunctional product is expressed which is not incorporated into virions. (ii) When constitutively expressed in a stable cell line without other viral proteins, gK is only incompletely processed. After superinfection with gK-deletion mutants, proper processin g is restored and mature gK is incorporated into,irions. (iii) The UL20 gen e product is specifically required for processing of gK. gK is not correctl y processed in a UL20 deletion mutant of PrV, and superinfection of gK-expr essing cells with PrV-UL20(-) does not restore processing. How ever, all ot her known structural viral glycoproteins appear to be processed normally in PrV-UL20(-)-infected cells. (iv) Coexpression of gK and UL20 restored gK p rocessing at least partially. Thus, our data show that the UL20 gene produc t is required for proper processing of PrV gK.