Temperature and pressure dependence of azurin stability as monitored by tryptophan fluorescence and phosphorescence. The case of F29A mutant

A novel hotspot of gelsolin instability triggers an alternative mechanism of amyloid aggregation

Gelsolin comprises six homologous domains, named G1 to G6. Single point substitutions in this protein are responsible for AGel amyloidosis, a hereditary disease causing progressive corneal lattice dystrophy, cutis laxa, and polyneuropathy. Although several different amyloidogenic variants of gelsolin have been identified, only the most common mutants present in the G2 domain have been thoroughly characterized, leading to clarification of the functional mechanism. The molecular events underlying the pathological aggregation of 3 recently identified mutations, namely A551P, E553K and M517R, all localized at the interface between G4 and G5, are here explored for the first time. Structural studies point to destabilization of the interface between G4 and G5 due to three structural determinants: β-strand breaking, steric hindrance and/or charge repulsion, all implying impairment of interdomain contacts. Such rearrangements decrease the temperature and pressure stability of gelsolin but do not alter its susceptibility to furin cleavage, the first event in the canonical aggregation pathway. These variants also have a greater tendency to aggregate in the unproteolysed forms and exhibit higher proteotoxicity in a C. elegans-based assay. Our data suggest that aggregation of G4G5 variants follows an alternative, likely proteolysis-independent, pathway.

Hands On: Using Tryptophan Fluorescence Spectroscopy to Study Protein Structure

Fluorescence spectroscopy is well suited to obtain information about the structure and function of proteins. The major advantage of this spectroscopic technique is the pronounced dependence of the fluorescence emission characteristics of fluorophores on their distinct local environment and the rather inexpensive equipment required. In particular, the use of intrinsic tryptophan fluorescence offers the possibility to study structure and function of proteins without the need to modify the protein. While fluorescence spectroscopy is technically not demanding, a number of factors can artificially alter the results. In this article, we systematically describe the most common applications in fluorescence spectroscopy of proteins, i.e., how to gain information about the local environment of tryptophan residues and how to employ changes in the environment to monitor an interaction with other substances. In particular, we discuss pitfalls and wrong and/or misleading interpretations of gained data together with potential solutions.

Polyubiquitin Drives the Molecular Interactions of the NF-κB Essential Modulator (NEMO) by Allosteric Regulation

The NF-κB essential modulator (NEMO) is the master regulator of NF-κB signaling, controlling the immune and nervous systems. NEMO affects the activity of IκB kinase-β (IKKβ), which relieves the inhibition of the NF-κB transcriptional regulation machinery. Despite major effort, there is only a very sparse, phenomenological understanding of how NEMO regulates IKKβ and shows specificity in its large range of molecular interactions. We explore the key molecular interactions of NEMO using a molecular biophysics approach, incorporating rapid-mixing stopped-flow, high-pressure, and CD spectroscopies. Our study demonstrates that NEMO has a significant degree of native structural disorder and that molecular flexibility and ligand-induced conformational change are at the heart of the molecular interactions of NEMO. We found that long chain length, unanchored, linear polyubiquitin drives NEMO activity, enhancing the affinity of NEMO for IKKβ and the kinase substrate IκBα and promoting membrane association. We present evidence that unanchored polyubiquitin achieves this regulation by inducing NEMO conformational change by an allosteric mechanism. We combine our quantitative findings to give a detailed molecular mechanistic model for the activity of NEMO, providing insight into the molecular mechanism of NEMO activity with broad implications for the biological role of free polyubiquitin.