title: A pH-dependent dimer lock in spider silk protein.
abstract: Spider dragline silk, one of the strongest polymers in nature, is composed of proteins termed major ampullate spidroin (MaSp) 1 and MaSp2. The N-terminal (NT) domain of MaSp1 produced by the nursery web spider Euprosthenops australis acts as a pH-sensitive relay, mediating spidroin assembly at around pH 6.3. Using amide hydrogen/deuterium exchange combined with mass spectrometry (MS), we detected pH-dependent changes in deuterium incorporation into the core of the NT domain, indicating global structural stabilization at low pH. The stabilizing effects were diminished or abolished at high ionic strength, or when the surface-exposed residues Asp40 and Glu84 had been exchanged with the corresponding amides. Nondenaturing electrospray ionization MS revealed the presence of dimers in the gas phase at pH values below--but not above--6.4, indicating a tight electrostatic association that is dependent on Asp40 and Glu84 at low pH. Results from analytical ultracentrifugation support these findings. Together, the data suggest a mechanism whereby lowering the pH to <6.4 results in structural changes and alteration of charge-mediated interactions between subunits, thereby locking the spidroin NT dimer into a tight entity important for aggregation and silk formation.
authors.affiliation: Division of Chemistry I, Department of Medical Biochemistry and Biophysics, Karolinska Institutet, SE-171 77 Stockholm, Sweden.
authors.forename: Michael
authors.initials: M
authors.lastname: Landreh
authors.forename: Glareh
authors.initials: G
authors.lastname: Askarieh
authors.forename: Kerstin
authors.initials: K
authors.lastname: Nordling
authors.forename: My
authors.initials: M
authors.lastname: Hedhammar
authors.forename: Anna
authors.initials: A
authors.lastname: Rising
authors.forename: Cristina
authors.initials: C
authors.lastname: Casals
authors.forename: Juan
authors.initials: J
authors.lastname: Astorga-Wells
authors.forename: Gunvor
authors.initials: G
authors.lastname: Alvelius
authors.forename: Stefan D
authors.initials: SD
authors.lastname: Knight
authors.forename: Jan
authors.initials: J
authors.lastname: Johansson
authors.forename: Hans
authors.initials: H
authors.lastname: Jörnvall
authors.forename: Tomas
authors.initials: T
authors.lastname: Bergman
mesh_terms: D000595:Amino Acid Sequence; D019943:Amino Acid Substitution; D000818:Animals; D003903:Deuterium; D019281:Dimerization; D005349:Fibroins / Q000737:chemistry* / Q000235:genetics; D006863:Hydrogen-Ion Concentration; D008958:Models, Molecular; D008969:Molecular Sequence Data; D016297:Mutagenesis, Site-Directed; D009994:Osmolar Concentration; D055503:Protein Multimerization; D055550:Protein Stability; D020836:Protein Structure, Quaternary; D017434:Protein Structure, Tertiary; D021122:Protein Subunits; D011994:Recombinant Proteins / Q000737:chemistry / Q000235:genetics; D021241:Spectrometry, Mass, Electrospray Ionization; D013112:Spiders / Q000737:chemistry / Q000235:genetics; D055672:Static Electricity; D014461:Ultracentrifugation
chemical_list: D021122:Protein Subunits; D011994:Recombinant Proteins; C086087:spidroin 1; D005349:Fibroins; D003903:Deuterium