The 57,000- to 65,000-dalton (Da)
Marek's disease herpesvirus A (
MDHV-A) antigen glycoprotein (gp57-65) has a 47,000-Da unglycosylated precursor
polypeptide (pr47), as determined by immunological detection after cell-free translation of infected-cell
mRNA. Cleavage of its
signal peptide yielded a 44,000-Da precursor
polypeptide molecule (pr44), detected both in vivo after
tunicamycin inhibition of glycosylation and in vitro after dog pancreas microsome processing of pr47. High-resolution pulse-chase studies showed that pr44 was quickly glycosylated (within 1 min) to nearly full size, a rapid processing time consistent with a cotranslational mode of glycosylation. This major glycosylation intermediate was further modified 6 to 30 min postsynthesis (including the addition of
sialic acid), and mature MDHV-A was secreted 30 to 120 min postsynthesis. Limited apparent secretion of pr44 occurred only in the first minute postsynthesis, in contrast to the later secretion of most of the MDHV-A
polypeptide as the fully glycosylated form described above. In addition, in the presence of
tunicamycin a small fraction of the newly synthesized MDHV-A
protein appeared as a secreted, partially glycosylated, heterogeneously sized precursor larger than pr44. pr44 constituted the major fraction of the new MDHV-A made in the presence of the inhibitor but the precursor was smaller than mature MDHV-A. These data indicate that there is a minor glycosylation pathway not sensitive to
tunicamycin and that "normal" glycosylation is not necessary for secretion. Collectively, the data demonstrate that the rapid release of most of the fully glycosylated form of MHDV-A from the cell shortly after synthesis is true secretion in a well-regulated and precisely programmed way and not the result of cell death and disruption.