Targeting of MAL, a putative element of the apical sorting machinery, to glycolipid-enriched membranes requires a pre-Golgi sorting event

The MAL proteolipid is a nonglycosylated polytopic membrane protein with sp ecific residence in glycolipid-enriched membrane (GEM) microdomains. MAL ha s been proposed as an element of the machinery for apical transport in pola rized epithelial cells. Previous work demonstrated that MAL requires its fo ur carboxyl-terminal amino acids to be targeted to GEMs. In the present wor k, we have engineered MAL with N-glycosylation consensus sequences to delim it the site at which commitment of MAL to access into GEMs takes place. Com parison of engineered MAL proteins bearing either an intact or a truncated carboxyl terminus revealed that whereas the former acquired endo H-sensitiv e and endo H-resistant mature glycosylation, the protein with a deleted car boxyl terminus did not. These results indicate that although MAL incorporat ion into GEMs takes place mainly in the Golgi, commitment of MAL to enter G EMs is a pre-Golgi event. (C) 1999 Academic Press.