Discovery and Characterization of an Aberrant Small Form of Glycoprotein I of Herpes Simplex Virus Type I in Cell Culture

The 380-to-393-amino-acid glycoprotein I (gI) encoded by herpes simplex virus 1 (HSV-1) is a critical mediator for viral cell-to-cell spread and syncytium formation. Here we report a previously unrecognized aberrant form of gI in HSV-1-infected cells. Production of this molecule is independent of cell type and viral strains. It had an unexpected gel migration size of approximately 23 kDa, was packaged into viral particles, and could be coimmunoprecipitated by antibodies to both N and C termini of gI. Deep sequencing failed to detect alternative RNA splicing, and the in vitro transcribed full-length mRNA gave rise to the 23 kDa protein in transfected cells. Combined mass spectrometry and antibody probing analyses detected peptide information across different regions of gI, suggesting the possibility of a full-length gI but with abnormal migration behavior. In line with this notion, the HA insertion mutagenesis revealed a stable fold in the gI extracellular region aa.38-196 resistant to denaturing conditions, whereas small deletions within this region failed the antibodies to detect the fast, but not the slow-moving species of gI. It is also intriguing that the structure could be perturbed to some extent by a gBsyn mutation, leading to exposure or shielding of the gI epitopes. Thus, the HSV-1 gI apparently adopts a very stable fold in its natural form, rendering it an unusual biophysical property. Our findings provide novel insight into the biological properties of HSV gI and have important implications in understanding the viral spread and pathogenesis. IMPORTANCE The HSV-1 gI is required for viral cell-to-cell spread within the host, but its behavior during infection has remained poorly defined. Along with the classic 66 kDa product, here we report a previously unrecognized, approximately 23 kDa form of gI. Biochemical and genetics analyses revealed that this molecule represents the full-length form of gI but adopts a stable fold in its extracellular domain that is resistant to denatured conditions, thus contributing to the aberrant migration rate. Our results revealed a novel property of HSV-1 gI and have important implications in understanding viral pathogenesis.