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Transmembrane domains (TMDs) are known as structural elements required for the insertion into the membrane of integral membrane proteins. We have provided here an example showing that the presence of the TMD is compulsory for the productive folding pathway of a membrane-anchored glycoprotein. Tyrosinase, a type I transmembrane protein whose insertion into the melanosomal membrane initiates melanin synthesis, is misfolded and degraded when expressed as a truncated polypeptide. We used constructs of tyrosinase ectodomain fused with chimeric TMDs or glycosylphosphatidylinositol anchor to gain insights into how the TMD enables the productive folding pathway of the ectodomain. We found that in contrast to the soluble constructs, the membrane-anchored chimeras fold into the native conformation, which allows their endoplasmic reticulum exit. They recruit calnexin to monitor their productive folding pathway characterized by the post-translational formation of buried disulfides. Lacking calnexin assistance, the truncated mutant is arrested in an unstable conformation bearing exposed disulfides. We showed that the transmembrane anchor of a protein may crucially, albeit indirectly, control the folding pathway of the ectodomain.

Original publication

DOI

10.1074/jbc.M603841200

Type

Journal article

Journal

J Biol Chem

Publication Date

04/08/2006

Volume

281

Pages

21682 - 21689

Keywords

Calnexin, Cell Line, Endoplasmic Reticulum, Glycosylphosphatidylinositols, Humans, Melanins, Membrane Proteins, Monophenol Monooxygenase, Protein Folding, Protein Structure, Tertiary, Protein Transport, Recombinant Fusion Proteins