PDBsum entry 2acm

Structural protein

PDB id

2acm

Contents

			Protein chains

					57 a.a.


					47 a.a.

References listed in PDB file

Key reference

Title

Autoproteolysis coupled to protein folding in the sea domain of the membrane-Bound muc1 mucin.

Authors

B.Macao, D.G.Johansson, G.C.Hansson, T.Härd.

Ref.

Nat Struct Mol Biol, 2006, 13, 71-76. [DOI no: 10.1038/nsmb1035]

PubMed id

16369486

Abstract

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.



	Figure 1. Figure 1. The human transmembrane mucins. Relative sizes of the different domains in transmembrane mucins and the sites for post-translational cleavage of the SEA (predicted for MUC16 and MUC17) and vWD (only found in MUC4) domains are shown. The PTS or mucin domains show considerable allelic variation in most of these mucins, and the total number of amino acids therefore varies. The MUC16 mucin is not fully N-terminally sequenced and the known size is not shown to scale (truncated as marked). SEA domains labeled "Cys pair" refer to sequences with two conserved cysteines positioned to form a disulfide.		Figure 3. Figure 3. SEA domain sequences and the structure of human MUC1 SEA. (a) Alignment of human SEA domains in mucins characterized by the GSVVV consensus sequence (dashed box) and examples of uncleaved mucin SEA domains. Sequence numbering and indicated secondary structure elements refer to the MUC1 SEA fragment studied here. Color coding represents surface-exposed side chains discussed in the text: magenta, Gly1097; blue, Ser1098; orange, surface-exposed hydrophobic residues conserved in mucin SEA domains; yellow, exposed hydrophobic residues conserved across species in MUC1; red, side chains in acidic region; cyan, asparagine residues subjected to N-linked glycosylation in MUC1. The conserved cysteine pair in uncleaved mucin SEA domains has been indicated by a line connecting boxed cysteine residues; other cysteines are also boxed. Sequence notation: h, human; m, mouse; SEA-1, SEA-2 and SEA-3, first, second and third SEA domains in MUC16 counted from the membrane domain toward the N terminus; rGP116, SEA domain in Ig-Hepta 7TM (non-mucin) protein; 1IVZ_111000814\|RIK, mouse MUC16 SEA homolog with known structure^18. The alignment was made using ClustalW28. (b) MUC1 SEA residues 1041-1144. The backbones of cleavage-site Gly1097 and Ser1098 residues are shown as magenta and blue sticks, respectively. Surface-exposed side chains discussed in the text are colored as in a. (c) Backbone superimposition of the ensemble of NMR structures. (d) Surface representation of MUC1 SEA. The view at left is as in b and c and the view at right is rotated by 180�. Color coding is as in a and b. Surfaces were calculated for all heavy atoms of residues 1041-1144 using a 1.4-� probe. (e) The site for autoproteolytic cleavage in MUC1 SEA. The view includes heavy atoms and polar hydrogens of residues within 10 � of the N' and C' termini resulting from autoproteolysis of the peptide bond between Gly1097 and Ser1098. Yellow, side chains of hydrophobic residues; orange, conserved and surface-exposed Phe1054 and Val1100 (as in a, b and d); dashed green lines, hydrogen bonds.

PROCHECK

	Headers