| 1. |
- Johansson, Denny, 1980
(författare)
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Autoproteolysis accelerated by conformational strain - a novel biochemical mechanism
- 2008
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Natural fragmentation of polypeptide chains by autoproteolysis occurs in a number of protein families. It is a vital step in the maturation of several enzymes and in the formation of membrane-associated mucins that constitute a part of the protective mucus barrier lining epithelial cells. These reactions follow similar routes involving an initial N-O or N-S acyl shift starting with a nucleophilic attack by a hydroxyl or thiol group on a carbonyl carbon followed by resolution of the ester intermediate. Previous studies indicate that distortion of the scissile peptide bond may play a role in autoproteolysis. Our structural, biochemical and molecular dynamics studies of the autoproteolyzed SEA domains from human membrane-bound mucin MUC1 and human orphan receptor GPR116 confirmed this by revealing a novel biochemical mechanism where the folding free energy accelerates cleavage by imposing conformational strain in the precursor structure. This mechanism may well be general for autoproteolysis. The structure of the cleaved MUC1 SEA domain was determined using NMR spectroscopy. It consists of four alpha-helices packed against the concave surface of a four-stranded anti-parallel beta-sheet. There are no disordered loops. The site of autoproteolysis is a conserved GSVVV sequence located at the ends of beta-sheets 2 and 3 where the resulting N- and C-terminal residues become integrated parts of these sheets after cleavage. The structure does not reveal any charge-relay system or oxyanion hole as would be expected if catalysis proceeded by way of transition state stabilization. The surface of the domain contains two hydrophobic patches that may serve as sites of interaction with other proteins, giving it a potential function in the regulation of the protective mucus layer. Combined studies of autoproteolysis and adoption of native fold show that these mechanisms proceed with the same rate and that the autoproteolysis has a global effect on structure. Studies of the stability and cleavage kinetics were performed by destabilizing core mutations or addition of denaturing co-solvents. Analysis revealed that ~7 kcal mol-1 of conformational free energy is partitioned as strain in the precursor. The results corroborate a mechanism where the autoproteolysis is accelerated by the concerted action of a conserved serine residue and strain imposed on the precursor structure upon folding, that is, the catalytic mechanism is substrate destabilization. The autoproteolysis of SEA is pH dependent. This is in line with a proposed mechanism with an initial N-O acyl shift, involving transient protonation of the amide nitrogen, and subsequent hydroxyl-mediated hydrolysis of the resulting ester. The mechanistic link between strain and cleavage kinetics is that strain induces a pyramidal conformation of the amide nitrogen which results in an increase of the pKa and thereby an acceleration of the N-O acyl shift. Furthermore we propose a water hydronium as proton donor in this step. This explains the absence of conserved acid-base functionality within the SEA structure.
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| 2. |
- Johansson, Denny, 1980, et al.
(författare)
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Protein autoproteolysis: conformational strain linked to the rate of peptide cleavage by the pH dependence of the N --> O acyl shift reaction.
- 2009
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Ingår i: Journal of the American Chemical Society. - : American Chemical Society (ACS). - 1520-5126 .- 0002-7863. ; 131:27, s. 9475-7
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Tidskriftsartikel (refereegranskat)abstract
- Nucleophilic attack by a side chain nucleophile on the adjacent peptide bond followed by N --> O or N --> S acyl shift is the primary step in protein autoproteolysis. Precursor structures of autoproteolytic proteins reveal strained (or twisted) amides at the site of cleavage, and we previously showed that SEA domain autoproteolysis involves substrate destabilization by approximately 7 kcal/mol. However, the precise chemical mechanism by which conformational energy is converted into reaction rate acceleration has not been understood. Here we show that the pH dependence of autoproteolysis in a slow-cleaving mutant (1G) of the MUC1 SEA domain is consistent with a mechanism in which N --> O acyl shift proceeds after initial protonation of the amide nitrogen. Unstrained amides have pK(a) values of 0 with protonation on the oxygen, and autoproteolysis is therefore immeasurably slow at neutral pH. However, conformational strain forces the peptide nitrogen into a pyramidal conformation with a significantly increased pK(a) for protonation. We find that pK(a) values of approximately 4 and approximately 6, as in model compounds of twisted amides, reproduce the rate of autoproteolysis in the 1G and wild-type SEA domains, respectively. A mechanism involving strain, nitrogen protonation, and N --> O shift is also supported by quantum-chemical calculations. Such a reaction therefore constitutes an alternative to peptide cleavage that is utilized in autoproteolysis, as opposed to a classical mechanism involving a structurally conserved active site with a catalytic triad and an oxyanion hole, which are not present at the SEA domain cleavage site.
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| 3. |
- Macao, Bertil, 1969, et al.
(författare)
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Autoproteolysis coupled to protein folding in the SEA domain of the membrane-bound MUC1 mucin.
- 2006
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Ingår i: Nature structural & molecular biology. - : Springer Science and Business Media LLC. - 1545-9993 .- 1545-9985. ; 13:1, s. 71-6
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Tidskriftsartikel (refereegranskat)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.
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| 4. |
- Pelaseyed, Thaher, 1979, et al.
(författare)
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Unfolding dynamics of the mucin SEA domain probed by force spectroscopy suggest that it acts as a cell-protective device.
- 2013
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Ingår i: The FEBS journal. - : Wiley. - 1742-4658 .- 1742-464X. ; 280:6, s. 1491-501
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Tidskriftsartikel (refereegranskat)abstract
- MUC1 and other membrane-associated mucins harbor long, up to 1 μm, extended highly glycosylated mucin domains and sea urchin sperm protein, enterokinase and agrin (SEA) domains situated on their extracellular parts. These mucins line luminal tracts and organs, and are anchored to the apical cell membrane by a transmembrane domain. The SEA domain is highly conserved and undergoes a molecular strain-dependent autocatalytic cleavage during folding in the endoplasmic reticulum, a process required for apical plasma membrane expression. To date, no specific function has been designated for the SEA domain. Here, we constructed a recombinant protein consisting of three SEA domains in tandem and used force spectroscopy to assess the dissociation force required to unfold individual, folded SEA domains. Force-distance curves revealed three peaks, each representing unfolding of a single SEA domain. Fitting the observed unfolding events to a worm-like chain model yielded an average contour length of 32 nm per SEA domain. Analysis of forces applied on the recombinant protein revealed an average unfolding force of 168 pN for each SEA domain at a loading rate of 25 nN·s(-1). Thus, the SEA domain may act as a breaking point that can dissociate before the plasma membrane is breached when mechanical forces are applied to cell surfaces.
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| 5. |
- Sandberg, Anders, 1975, et al.
(författare)
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SEA domain autoproteolysis accelerated by conformational strain: energetic aspects.
- 2008
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Ingår i: Journal of molecular biology. - : Elsevier BV. - 1089-8638 .- 0022-2836. ; 377:4, s. 1117-29
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Tidskriftsartikel (refereegranskat)abstract
- A subclass of proteins with the SEA (sea urchin sperm protein, enterokinase, and agrin) domain fold exists as heterodimers generated by autoproteolytic cleavage within a characteristic G(-1)S+1VVV sequence. Autoproteolysis occurs by a nucleophilic attack of the serine hydroxyl on the vicinal glycine carbonyl followed by an N-->O acyl shift and hydrolysis of the resulting ester. The reaction has been suggested to be accelerated by the straining of the scissile peptide bond upon protein folding. In an accompanying article, we report the mechanism; in this article, we provide further key evidence and account for the energetics of coupled protein folding and autoproteolysis. Cleavage of the GPR116 domain and that of the MUC1 SEA domain occur with half-life (t((1/2))) values of 12 and 18 min, respectively, with lowering of the free energy of the activation barrier by approximately 10 kcal mol(-1) compared with uncatalyzed hydrolysis. The free energies of unfolding of the GPR116 and MUC1 SEA domains were measured to approximately 11 and approximately 15 kcal mol(-1), respectively, but approximately 7 kcal mol(-1) of conformational energy is partitioned as strain over the scissile peptide bond in the precursor to catalyze autoproteolysis by substrate destabilization. A straining energy of approximately 7 kcal mol(-1) was measured by using both a pre-equilibrium model to analyze stability and cleavage kinetics data obtained with the GPR116 SEA domain destabilized by core mutations or urea addition, as well as the difference in thermodynamic stabilities of the MUC1 SEA precursor mutant S1098A (with a G(-1)A+1VVV motif) and the wild-type protein. The results imply that cleavage by N-->O acyl shift alone would proceed with a t((1/2)) of approximately 2.3 years, which is too slow to be biochemically effective. A subsequent review of structural data on other self-cleaving proteins suggests that conformational strain of the scissile peptide bond may be a common mechanism of autoproteolysis.
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