Specific isoforms of the resident endoplasmic reticulum protein glucosidase II associate with the CD45 protein-tyrosine phosphatase via a lectin-like interaction

We have previously demonstrated that CD45 physically associates with the endoplasmic reticulum processing enzyme glucosidase II (GII). GII consists of the catalytic alpha-chain and an associated beta-chain. To gain insight into the basis of the association between CD45 and GII, we examined the biochemical requirements for the interaction. We show that the alpha-subunit is essential for the interaction. Interestingly, only a higher molecular weight form of GII alpha! is capable of associating with CD45 in a competitive situation where multiple GII alpha isoforms are expressed. Further, transfection studies demonstrate that only isoforms containing the alternatively spliced sequence Box Al are capable of binding CD45, although all isoforms are catalytically active, The interaction between CD45 and GII is dependent on the active site of GII, is mediated through the carbohydrate on CD45, and can be inhibited with mannose. Taken together, these results suggest that GII alpha acts as a lectin and binds to CD45 in an exon-dependent manner. This lectin activity of GIT may be a novel mechanism for the regulation of CD45 biology and play a role in immune function, possibly by regulating CD45 glycosylation.