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. Author manuscript; available in PMC: 2016 Jan 1.
Published in final edited form as: Curr Protein Pept Sci. 2015;16(1):31–48. doi: 10.2174/1389203716666150213160438

Figure 3. Proposed models for GIIβ MRH domain-mediated enhancement of N-glycan deglucosylation.

Figure 3

GII is a heterodimer composed of a GIIα catalytic and a GIIβ regulatory subunit. GIIβ enhances GII deglucosylation activity towards both G2M9 and G1M9 through its MRH domain. Upon binding mannose units in the B and/or C arms of the glycan, the GIIβ MRH domain presents bonds to be cleaved to the GIIα catalytic site (star). GIIβ also provides the retention/retrieval signal for proper ER localization of the heterodimer (−ValAspGluLeu (VDEL) in S. pombe). G2B domain is involved in GIIα-GIIβ interaction. Residues Gln-384, Arg-414, Glu-433, Tyr-439 (QREY) form the binding pocket that is the “signature motif” for MRH domain-containing proteins. There are two possible models for the role of Trp-409 in GII activity: In (A) mannose-binding essential residues Gln-384, Arg-414, Glu-433, Tyr-439 form a pocket which binds arm C of the glycan, while residue Trp-409 (W) interacts with arm B. This bidentate interaction allows the glucose-containing arm A to be juxtaposed to GIIα’s catalytic site. In (B) Trp-409 interacts with other regions of the β-subunit and influences its affinity for N-glycans. These models suggest that removal of mannoses by ER mannosidases will reduce both the binding of the glycan and GII activity.