Determination of the "minimal glycosylation distance" (MGD). (A) The MGD is defined as the minimum number of residues required to bridge the distance between the lumenal end of a transmembrane segment (white) and the active site of oligosaccharyl transferase (Y denotes a glycosylated site, Φ a non-glycosylated site). (B) MGD measurements for constructs with the indicated model segments. Half-maximal glycosylation (40%) is indicated by the horizontal line. The MGD value is counted from the first polar residue before the transmembrane segment (Gln) up to the acceptor Asn residue as in .