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Structural mass spectrometry encompasses a suite of MS-based techniques that provide information about protein structure, interactions, and dynamics. Native MS preserves non-covalent complexes in the gas phase, measuring the intact mass and stoichiometry of protein assemblies. Cross-linking MS (XL-MS) identifies residue pairs in spatial proximity by chemically cross-linking them and identifying the cross-linked peptides by LC-MS/MS, providing distance constraints analogous to NMR NOEs but applicable to much larger systems. These approaches complement high-resolution structural methods by characterizing heterogeneous, dynamic, and transient interactions that resist crystallization, providing restraints for integrative structural modeling.
The traditional structural biology methods — crystallography, cryo-EM, NMR — each have blind spots. Crystallography requires crystals, cryo-EM struggles with small or heterogeneous specimens, and NMR is limited to small proteins. Structural mass spectrometry fills these gaps by providing structural information from heterogeneous, dynamic, and even in-cell samples, with no size limit and minimal sample requirements.
Native mass spectrometry uses gentle electrospray ionization to transfer intact protein complexes from solution to the gas phase without disrupting non-covalent interactions. The measured mass-to-charge ratio reveals the total mass of the complex, from which stoichiometry can be deduced. For a heteromeric complex, native MS answers a fundamental question that can be surprisingly difficult to answer otherwise: how many copies of each subunit are present? Collision-induced dissociation (CID) — gradually increasing the collision energy to disassemble the complex in the gas phase — reveals which subunits are peripheral (ejected first) and which are core (released last), providing topology information. Native MS has characterized complexes from small heterodimers to intact ribosomes and virus capsids.
Cross-linking mass spectrometry (XL-MS) chemically bridges residue pairs that are in spatial proximity. A bifunctional cross-linker (like DSS or BS3, which reacts with primary amines on lysine side chains) is added to the protein or complex in solution. Cross-links form between lysine pairs within ~25-30 Angstroms. The cross-linked protein is then digested with protease, and the resulting peptide mixture is analyzed by LC-MS/MS. Specialized software (like pLink, XlinkX, or xiSearch) identifies cross-linked peptide pairs from their characteristic fragmentation patterns. Each identified cross-link is a distance constraint — the two residues must be within cross-linkable distance in the native structure.
The power of structural MS is most apparent in integrative structural biology — combining data from multiple techniques to model structures that no single method can determine alone. The nuclear pore complex, the mediator complex, and chromatin remodeling machines have all been structurally characterized using integrative approaches where XL-MS distance constraints guide the assembly of subunit crystal structures into the overall complex architecture, constrained by cryo-EM density maps and SAXS shape information. Structural MS provides the "glue" that connects high-resolution substructures into the complete biological assembly.