The  - and  -subunits are required for expression of catalytic activity in the hetero-dimeric glucosidase II complex from human liver

The [alpha]- and [beta]-subunits of the hetero-dimeric glucosidase II complex from human liver were cloned and expressed in COS-1 cells. The 4106 bp full-length cDNA for the [alpha]-subunit contained a 2835 bp ORF encoding a 107 kDa polypeptide. The 2095 bp cDNA for the [beta]-subunit encodes a ~60 kDa protein in a continuous 1605 bp ORF. The [alpha]- and [beta]-subunits each contain two potential Asn-Xaa-Thr/Ser acceptor sites, with only one site in the [alpha]-subunit (Asn97) being glycosylated. Additional [lambda]-clones were isolated for each subunit containing in-frame insertions/deletions within the coding region, indicating alternative splicing. Analysis of different human tissues revealed ~4.4 kb and ~2.4 kb transcripts for [alpha]- and [beta]-subunit, respectively, consistent with their full-length cDNA. Coexpression of the [alpha]- and [beta]-subunits in COS-1 cells resulted in >4-fold increase of glucosidase II activity. An inactive protein was obtained, however, after transfection with the [alpha]-subunit alone, showing that both subunits are essential for expression of active glucosidase II. The observation that the enzyme, previously purified from pig liver and lacking the [beta]-subunit, was catalytically active indicates that the [beta]-subunit is involved in [alpha]-subunit maturation rather than being required for enzymatic activity once the [alpha]-subunit has acquired its mature form. The [alpha]-subunit is expressed in COS-1 cells as an ER-located protein, whether inactive or part of a catalytically active complex. This suggests that ER-localization of the [alpha]-subunit, when associated with the dimeric enzyme complex, is mediated by the C-terminal HDEL-signal in the [beta]-subunit, whereas the apparently incompletely folded form of the inactive [alpha]-subunit could be retained in the ER by the putative "glycoprotein-specific quality control machinery."