Trimming of glucoses from N-linked core glycans on newly synthesized glycoproteins occurs sequentially through the action of Glucosidases I and II in the endoplasmic reticulum (ER). Glucosidase II is an ER-localized enzyme that contains a and b subunits (Glucosidase IIa and Glucosidase IIb) which form a defined heterodimeric complex. Glucosidase IIa is the catalyitc core of the enzyme and can function independently of the b subunit. The sequence of Glucosidase IIb encodes protein rich in glutamic and aspartic acid with a putative ER retention signal (HDEL) at the C-terminus. The phosphorylated form of Glucosidase IIb is localized in the plasma membrane and is highly expressed in FGF-stimulated fibroblasts and epidermal carcinoma cells. Glucosidase IIb was first purified from a human carcinoma cell line as a potential substrate for protein kinase C. Through the HDEL signal at the C-terminus, Glucosidase IIb retains the complete complex in the ER.