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Autoproteolysis coupled to protein folding in the SEA domain of the membrane-bound MUC1 mucin


The single cell layer of the lungs and the gastrointestinal tract is protected by the mucus formed by large glycoproteins called mucins. Transmembrane mucins typically contain 110-residue SEA domains located next to the membrane. These domains undergo post-translational cleavage between glycine and serine in a characteristic GSVVV sequence, but the two peptides remain tightly associated. We show that the SEA domain of the human MUC1 transmembrane mucin undergoes a novel type of autoproteolysis, which is catalyzed by conformational stress and the conserved serine hydroxyl. We propose that self-cleaving SEA domains have evolved to dissociate as a result of mechanical rather than chemical stress at the apical cell membrane and that this protects epithelial cells from rupture. We further suggest that the cell can register mechanical shear at the mucosal surface if the dissociation is signaled via loss of a SEA-binding protein.

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Figure 1: The human transmembrane mucins.
Figure 2: SDS-PAGE and thermal-stability analyses of wild-type and mutant MUC1 SEA samples.
Figure 3: SEA domain sequences and the structure of human MUC1 SEA.
Figure 4: NMR spectroscopy of wild-type SEA and uncleaved SEA mutants.

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This work was supported in part by grants from the Swedish Research Council, the Hasselblad Foundation and the Wallenberg Foundation (to the Swedish NMR Centre).

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Correspondence to Torleif Härd.

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Supplementary information

Supplementary Fig. 1

SDS-PAGE analyses of wild-type and mutant MUC1 SEA samples. (PDF 416 kb)

Supplementary Fig. 2

Circular dichroism spectra. (PDF 150 kb)

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Macao, B., Johansson, D., Hansson, G. et al. Autoproteolysis coupled to protein folding in the SEA domain of the membrane-bound MUC1 mucin. Nat Struct Mol Biol 13, 71–76 (2006).

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