The role of Golgi [alpha]-mannosidase IIx in glycan processing
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N-linked glycoproteins are important in diverse cellular processes in mammals including development, self/non-self recognition and inflammation. At a molecular level, involvement of N-glycoproteins in these processes is seen as N-glycans regulating protein folding, targeting, recognition and adhesion processes. Studies employing a wide array of approaches such as animal models and mammalian cell lines have revealed that the class II mannosidases (CAZy family GH38), Golgi α-mannosidase II and Golgi α-mannosidase IIx, play an essential role in the conversion of hybrid-type N-glycans to complex-type N-glycans. Golgi α-mannosidase IIx is expressed as multiple transcript splice variants, only a subset of which encode functionally active enzymes in cell culture. The functional role for each of the splice variants is still unclear. We have expressed a recombinant, soluble form of Golgi α-mannosidase IIx that can cleave the hybrid-type native oligosaccharide substrate, GlcNAcMan5GlcNAc2-PA to GlcNAcMan3GlcNAc2-PA, is unable to cleave high-mannose native oligosaccharide substrates, is activated 2.5 fold by cobalt, and is not detectable in mouse liver and kidney extracts as determined by immunoblotting. These data suggest that Golgi α-mannosidase II and Golgi α-mannosidase IIx catalyze functionally redundant pathways in the Golgi apparatus.