L273S missense substitution in human lysosomal acid lipase creates a new N-glycosylation site.

Human lysosomal acid lipase (LAL), when expressed in HeLa cells using the Vaccinia T7 expression system, showed four major molecular forms ranging from 42 to 54 kDa. Treatment with endoglycosidase H resulted in a 42 kDa protein, indicating that the molecular weight variations were due to N-glycosylation. A missense substitution, L273S, previously detected in a patient with cholesteryl ester storage disease (CESD), produced catalytically inactive LAL showing a largest molecular mass form of 56 kDa instead of 54 kDa. Analysis of the amino acid sequence in the close proximity of the mutation (NMS- NML) indicated that the L273S mutation creates an additional N-glycosylation consensus (N-X-S/T) in this region. Two site directed mutants disrupting this consensus, QMS and QML, when expressed in HeLa cells, did not show the 56 kDa form but the normal 54 kDa band whereas deglycosylation always resulted in the major 42 kDa form, as observed with normal LAL and the L273S mutant. These data confirmed that an additional N-glycosylation at N271 was responsible for the 56 kDa form of the protein produced from the L273S allele. Furthermore, deglycosylation of normal LAL reduced the acid hydrolase activity towards both tri-oleyl glycerol and cholesteryl oleate by 50%, strongly suggesting that N-linked carbohydrate residues are important for optimal catalytic activity.