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## Acknowledgements

T.M.W. was supported by a PhD studentship from the Wellcome Trust (096601/Z/11/Z) and M.L. by an MRC research grant (MR/N000366/1). We thank the Wellcome Trust for equipment grant support to the EM Facility. The Bioimaging Facility microscopes used in this study were purchased with grants from BBSRC, Wellcome and the University of Manchester Strategic Fund. Special thanks go to Peter March for his help with the light microscopy and Mike Jackson from the Flow Cytometry Core Facility for his help and advice. The York Centre of Excellence in Mass Spectrometry was created thanks to a major capital investment through Science City York, supported by Yorkshire Forward with funds from the Northern Way Initiative. U.K. received funding from the EU (E-Rare project EURO-CDG-2) and from the German Federal Ministry of Education and Research (BMBF) (DIMEOs (1EC1402B)). We give thanks to various colleagues for generously providing reagents as indicated in the Methods section, and thank Professor Philip Woodman (University of Manchester) for comments on the manuscript.

## Author contributions

M.L. managed the project and together with T.M.W. conceptualized the experiments. T.M.W. performed all the experiments apart those described below. T.M.W. and M.L. analyzed the data. T.M.W. prepared the figures for the manuscript. M.L. wrote the manuscript with input from all co-authors. M.T. helped conceive the project and provided patient fibroblasts. W.L.C. and U.K. generated the GORAB knockout mouse and prepared tissues from it. M.J. and E.J. generated and analyzed the immuno-EM and fibroblast EM data. A.A.M. performed the EM analysis of the HRP-ST cells. E.P., J.T.-O. and D.U. generated and analyzed the glycomics data for fibroblast cells. W.M. performed the glycan analysis of the GORAB knockout mouse samples. A.P.M. helped perform the surface plasmon resonance experiments. C.B. and Y.G. made and provided expertise in the use of alkyne-tagged sialic acid. M.R. and F.T.W. provided assistance with the liposome experiments.

## Additional information

Supplementary Information accompanies this paper at https://doi.org/10.1038/s41467-018-08044-6.

**Competing interests:** The authors declare no competing interests.

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