Control of glycosylation of MHC classII-associated invariant chain by translocon-associated RAMP4. Schröder Katja, Martoglio Bruno, Hofmann Michael, Hölscher Christina, Hartmann Enno, Prehn Siegfried, Rapoport Tom A., Dobberstein Bernhard. The EMBO Journal. 1999;18:4804–4815. doi: 10.1093/emboj/18.17.4804.
Control of glycosylation of MHC classII-associated invariant chain by translocon-associated RAMP4
Katja Schröder, Bruno Martoglio, Michael Hofmann, Christina Hölscher, Enno Hartmann, Siegfried Prehn, Tom A.Rapoport, Bernhard Dobberstein
The EMBO Journal, 18, 4804–4815, 1999
In this paper we had identified a region in nascent invariant chain (Ii) that could be cross-linked to RAMP4 during translocation across the endoplasmic reticulum membrane. Mutants in Ii that prevented cross-linking to RAMP4 showed reduced glycosylation. Based on these observations, we had proposed that the RAMP4 interaction with Ii affects its N-glycosylation. Recently we have discovered that the reduced level of N-glycosylation of the Ii mu tants is due to an enzymatic activity occurring during the native immunoprecipitation. When cells expressing the mutant invariant chains are lysed in the presence of N-ethylmaleimide (NEM), or denatured prior to immunoprecipitation, no reduction in N-glycosylation is observed in the mutants relative to the wild-type invariant chain. This NEM-sensitive activity most likely results from the cytoplasmic N-glycanase (Suzuki et al., 1995). The differential sensitivity of the wild-type Ii protein and the mutants may be due to a conformational change leading to increased accessibility of Ii mutants to the glycanase (Ii mutants affecting the RAMP4 interaction are in a region of Ii implicated in trimerization). In conclusion, our data do not support a role for RAMP4 in mediating the co-translational N-glycosylation of Ii. Rather, mutants affecting the RAMP4 interaction must be interpreted as affecting Ii conformation or assembly. Thus, the functional significance of the persistent co-translational interaction between the invariant chain and RAMP4 remains to be elucidated.
Jeannie Barrett, Klaus Meese and Bernhard Dobberstein
References
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