[en] All gammaherpesviruses encode a glycoprotein positionally homologous to the Epstein-Barr virus gp350 and the Kaposi's Sarcoma associated herpesvirus (KSHV) K8.1. In this study, we characterized that of Bovine Herpesvirus-4 (BoHV-4), encoded by the Bo10 gene. We identified a 180 kDa gene product, gp180, which was incorporated into the virion envelope. A Bo10 deletion virus was viable, but showed a growth deficit associated with reduced binding to epithelial cells. This seemed to reflect an interaction of gp180 with glycosaminoglycans (GAGs), since the Bo10 mutant was both less infectious for GAG(+) cells than the wild-type and more infectious for GAG(-) cells. However, we could not identify a direct interaction between gp180 and GAGs, implying that any direct interaction must be of low affinity. This function of gp180 was very similar to that previously identified for the Murid Herpesvirus 4 gp150, and also to the Epstein-Barr virus gp350 that promotes CD21(+) cell infection and inhibits CD21(-) cell infection. We propose that such proteins generally regulate virion attachment both by binding to cells and by covering another receptor-binding protein until they are displaced. Thus they regulate viral tropism both positively and negatively depending upon the presence or absence of their receptor.