Acidic Residues Control the Dimerization of the N-terminal Domain of Black Widow Spiders' Major Ampullate Spidroin 1.

Dragline silk is the most prominent amongst spider silks and comprises two types of major ampullate spidroins (MaSp) differing in their proline content. In the natural spinning process, the conversion of soluble MaSp into a tough fiber is, amongst other factors, triggered by dimerization and conformational switching of their helical amino-terminal domains (NRN). Both processes are induced by protonation of acidic residues upon acidification along the spinning duct. Here, the structure and monomer-dimer-equilibrium of the domain NRN1 of Latrodectus hesperus MaSp1 and variants thereof have been investigated, and the key residues for both could be identified. Changes in ionic composition and strength within the spinning duct enable electrostatic interactions between the acidic and basic pole of two monomers which prearrange into an antiparallel dimer. Upon naturally occurring acidification this dimer is stabilized by protonation of residue E114. A conformational change is independently triggered by protonation of clustered acidic residues (D39, E76, E81). Such step-by-step mechanism allows a controlled spidroin assembly in a pH- and salt sensitive manner, preventing premature aggregation of spider silk proteins in the gland and at the same time ensuring fast and efficient dimer formation and stabilization on demand in the spinning duct.

Structural transitions in full-length human prion protein detected by xenon as probe and spin labeling of the N-terminal domain.

Fatal neurodegenerative disorders termed transmissible spongiform encephalopathies (TSEs) are associated with the accumulation of fibrils of misfolded prion protein PrP. The noble gas xenon accommodates into four transiently enlarged hydrophobic cavities located in the well-folded core of human PrP(23-230) as detected by [(1)H, (15)N]-HSQC spectroscopy. In thermal equilibrium a fifth xenon binding site is formed transiently by amino acids A120 to L125 of the presumably disordered N-terminal domain and by amino acids K185 to T193 of the well-folded domain. Xenon bound PrP was modelled by restraint molecular dynamics. The individual microscopic and macroscopic dissociation constants could be derived by fitting the data to a model including a dynamic opening and closing of the cavities. As observed earlier by high pressure NMR spectroscopy xenon binding influences also other amino acids all over the N-terminal domain including residues of the AGAAAAGA motif indicating a structural coupling between the N-terminal domain and the core domain. This is in agreement with spin labelling experiments at positions 93 or 107 that show a transient interaction between the N-terminus and the start of helix 2 and the end of helix 3 of the core domain similar to that observed earlier by Zn(2+)-binding to the octarepeat motif.

Resonance assignment of an engineered amino-terminal domain of a major ampullate spider silk with neutralized charge cluster.

Spider dragline fibers are predominantly made out of the major ampullate spidroins (MaSp) 1 and 2. The assembly of dissolved spidroin into a stable fiber is highly controlled for example by dimerization of its amino-terminal domain (NRN) upon acidification, as well as removal of sodium chloride along the spinning duct. Clustered residues D39, E76 and E81 are the most highly conserved residues of the five-helix bundle, and they are hypothesized to be key residues for switching between a monomeric and a dimeric conformation. Simultaneous replacement of these residues by their non-titratable analogues results in variant D39N/E76Q/E81Q, which is supposed to fold into an intermediate conformation between that of the monomeric and the dimeric state at neutral pH. Here we report the resonance assignment of Latrodectus hesperus NRN variant D39N/E76Q/E81Q at pH 7.2 obtained by high-resolution triple resonance NMR spectroscopy.

An inhibitory peptide derived from the α-subunit of the epithelial sodium channel (ENaC) shows a helical conformation.

Proteolytic activation of the heteromeric epithelial sodium channel (ENaC) is thought to involve the release of inhibitory peptides from the extracellular domains of its α- and γ-subunit. Recently, we demonstrated that an α-13-mer peptide, corresponding to a putative inhibitory region within the extracellular domain of human αENaC, inhibits human αßγENaC. The aim of the present study was to investigate the structural basis of the inhibitory effect of this α-13-mer peptide. Analysis of the peptide by replica exchange molecular dynamics method, circular dichroism spectroscopy, nuclear magnetic resonance spectroscopy, and molecular dynamics simulations suggested that a helical turn at the carboxy-terminus is the preferred conformational state of the α-13-mer peptide. From this we predicted that a specific mutation (leucine 188 to alanine) should have a strong effect on the conformational preferences of the peptide. To functionally test this, we compared the effect of the wild-type α-13-mer with that of a mutant α-L188A-13-mer on ENaC currents in Xenopus laevis oocytes heterologously expressing human αßγENaC. We demonstrated that replacing the leucine 188 by alanine abolished the inhibitory effect of the α-13-mer peptide on ENaC. These findings suggest that a helical conformation in its carboxyterminal part is functionally important to mediate ENaC inhibition by the α-13-mer peptide. However, high resolution structural information on the complex of the inhibitory αENaC peptide and the channel are needed to confirm this conclusion.

Energy landscape of the prion protein helix 1 probed by metadynamics and NMR.

The characterization of the structural dynamics of proteins, including those that present a substantial degree of disorder, is currently a major scientific challenge. These dynamics are biologically relevant and govern the majority of functional and pathological processes. We exploited a combination of enhanced molecular simulations of metadynamics and NMR measurements to study heterogeneous states of proteins and peptides. In this way, we determined the structural ensemble and free-energy landscape of the highly dynamic helix 1 of the prion protein (PrP-H1), whose misfolding and aggregation are intimately connected to a group of neurodegenerative disorders known as transmissible spongiform encephalopathies. Our combined approach allowed us to dissect the factors that govern the conformational states of PrP-H1 in solution, and the implications of these factors for prion protein misfolding and aggregation. The results underline the importance of adopting novel integrated approaches that take advantage of experiments and theory to achieve a comprehensive characterization of the structure and dynamics of biological macromolecules.