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. 2015 Dec 15;1(9):475–476. doi: 10.1021/acscentsci.5b00390

Structure from Splatter

Julie A Leary 1,a
PMCID: PMC4827561  PMID: 27163012

The use of mass spectrometry (MS) to determine native protein structure has become an invaluable tool for protein biochemists. Although initially met with extreme skepticism, it is now very clear that the complementary use of “native condition” MS with dissociative MS techniques and molecular modeling has provided specific structural information for multimeric protein assemblies and subunit interaction sites.1−5

Early pioneering studies by the groups of Carol Robinson and Albert Heck helped pave the way for more recent studies by Wysocki and co-workers who introduced surface induced dissociation (SID) in combination with ion mobility MS for native protein structure determination.6,7 The refinement of SID and its development in conjunction with ion mobility mass spectrometry (IMS) by the Wysocki group have brought this particular dissociation technique into the mainstream of MS for biomolecule structure determination.8−10

The methodology of SID simply described involves accelerating an analyte ion toward a specific surface under high vacuum conditions.11,12 During impact, the ions fragment (small organic molecule such as a peptide) or dissociate (multiprotein assemblies). In many instances, the results of SID are far more specific than collision induced dissociation (CID) given the higher energies involved with large surfaces, and therefore it can often provide unambiguous information on amino acid modifications, peptide sequence, and noncovalent interaction sites. In the ACS Central Science article by the Wysocki group, the authors show that the protein assembly toyocamycin nitrile hydratase (TNH) is a protein dimer of two identical trimers each containing alpha, beta, and gamma subunits (Figure 1). The general approach used for analyzing such quarternary multisubunit protein assemblies is to first buffer exchange the sample into a mass spectrometer appropriate buffer such as ammonium acetate and obtain the molecular mass of the intact complex. Once this is established, a variety of organic solvents at different concentrations can be used to gently disrupt the complex, thus allowing one to determine the number and mass of the subunits. This solution disruption methodology is also used to probe which subunits might be more easily lost from the complex, thus giving some information on location of the subunits relative to each other. It can also be used to identify which subunits may be interacting with each other.

Figure 1.

Figure 1

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Two possible hetero-hexomeric structures of TNH. Figure reproduced with permission from ref (6). Copyright 2015 American Chemical Society.

SID of the disrupted complexes, in combination with IMS, generates maps of drift time vs mass to charge ratio, which then allows the investigator to place the subunits in space relative to each other and determine if and how they are interacting with each other. The collision cross sections generated from the drift times of the SID generated species are probed with MOBCAL and various homology modeling and molecular dynamics calculations. In cases where further refinement is required to determine the exact locations of interactions sites between subunits, chemical cross-linking and site specific labeling techniques are used.

The TNH hexamer complex is quite challenging given the fact that the assembly is composed of two identical dimers made up of three different subunits, alpha beta and gamma. Unless the nanoelectrospray conditions, buffer, solvents system, and voltages are carefully chosen and calibrated with known quarternary structure protein assemblies, the complex might incorrectly be measured as consisting of only one heterologous trimer. Equally important is the fact that the authors carefully and thoroughly conduct both chemical cross-linking and site specific modifications in order to validate the specific interaction sites between the subunits.

Linking SID to an ion mobility instrument was exceptionally forward-looking and brave. This is not a case of making a better mouse trap, but rather taking the initiative and convincing the chemical community that dissociating these biomolecular complexes via a surface rather than a gas is much more efficient.

References

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