ENDOPLASMIC-RETICULUM GLUCOSIDASE-II IS COMPOSED OF A CATALYTIC SUBUNIT, CONSERVED FROM YEAST TO MAMMALS, AND A TIGHTLY BOUND NONCATALYTIC HDEL-CONTAINING SUBUNIT

Trimming of glucoses from N-linked core glycans on newly synthesized g lycoproteins occurs sequentially through the action of glucosidases I and II in the endoplasmic reticulum (ER), We isolated enzymatically ac tive glucosidase II from rat liver and found that, in contrast with pr evious reports, it contains two subunits (alpha and beta), Sequence an alysis of peptides derived from them allowed us to identify their corr esponding human cDNA sequences, The sequence of the alpha subunit pred icted a soluble protein (104 kDa) devoid of known signals for residenc e in the ER, It showed homology with several other glucosidases but no t with glucosidase I. Among the homologues, we identified a Saccharomy ces cerevisiae gene, which we showed by gene disruption experiments to be the functional catalytic subunit of glucosidase II. The disrupted yeast strains had no detectable growth defect, The sequence of the bet a subunit (58 kDa) showed no sequence homology with other known protei ns, It encoded a soluble protein rich in glutamic and aspartic acid wi th a putative ER retention signal (HDEL) at the C terminus, This sugge sted that the beta subunit is responsible for the ER localization of t he enzyme.