Soluble G protein variant sG of RSV is N- and O-glycosylated, and the leading 19 amino acids, which are assumed to be the signal sequence directing nascent sG to the endoplasmic reticulum (ER) for glycosylation (Roberts et al, 1994), are cleaved off. While the responsible peptidase has not been experimentally determined, the signal peptidase complex (SPC) located at the endoplasmic reticulum membrane is a candidate as it cleaves human protein signal peptides and is known to be involved in cleavage of signal peptides in other viruses (Liaci et al. 2021). This complex can exist in two forms, SPC-A and SPC-C, differing in their catalytic subunits (SEC11A in SPC-A, and SEC11C in SPC-C).