Extracting structural information from charge-state distributions of intrinsically disordered proteins by non-denaturing electrospray-ionization mass spectrometry

Abstract

Intrinsically disordered proteins (IDPs) exert key biological functions but tend to escape identification and characterization due to their high structural dynamics and heterogeneity. The possibility to dissect conformational ensembles by electrospray-ionization mass spectrometry (ESI-MS) offers an attracting possibility to develop a signature for this class of proteins based on their peculiar ionization behavior. This review summarizes available data on charge-state distributions (CSDs) obtained for IDPs by non-denaturing ESI-MS, with reference to globular or chemically denatured proteins. The results illustrate the contributions that direct ESI-MS analysis can give to the identification of new putative IDPs and to their conformational investigation.

Keywords: IDP compaction; Sic1; charge-state distributions; charge-to-mass relation; intrinsically folded structural units; native mass spectrometry; protein folding; proteomics; solvent-accessible surface area; α-synuclein.

Figure 1. Examples of CSDs obtained by nano-ESI-MS under non-denaturing conditions. The spectra were recorded on a hybrid, quadrupole time-of-flight mass spectrometer (QSTAR Elite, AB-Sciex). Samples were infused at room temperature, by metal-coated borosilicate capillaries with emitter tips of 1 μm internal diameter (Proxeon). The following instrumental settings were applied: declustering potential 60/80 V, ion spray voltage 1.1/1.2 kV, curtain-gas pressure 20 psi. (A) 12 μM β-lactoglobulin in 10 mM ammonium acetate, pH 7.0. (B) 12 μM α-synuclein in 10 mM ammonium acetate, pH 7.4 (negative-ion mode). (C) 15 μM Sic1 in 50 mM ammonium acetate, pH 6.5. (D) 10 μM Sic1^215–284 in 50 mM ammonium acetate, pH 6.5.

Figure 2. Charge-to-mass plot for distinct components of some IDPs analyzed by non-denaturing ESI-MS. In the case of bimodal distributions, each component is represented by a circle, colored according to the relative abundance as specified in the inserted table. For unimodal distributions (unique case of full-length Sic1), the global average charge is considered and the symbol is colored in black. The gray small squares and triangles represent data for globular proteins, respectively under non-denaturing and denaturing conditions. The gray lines are interpolations by power-law functions.