The viral glycoprotein precursor GPC is post-translationally proteolytically processed by the S1P cellular protease to yield the two mature virion glycoproteins GP-1 (40-46 kDa) and GP-2 (35 kDa) (Beyer et al., 2003; Lenz et al., 2001; Pinschewer et al., 2003b) .