Top-down mass spectrometry of integral membrane proteins

Abstract

Top-down mass spectrometry focuses on intact proteins, thereby avoiding loss of information accompanying ‘shotgun’ protocols that reduce the proteome to a collection of peptides. A suite of liquid-chromatography technologies has been developed for purification of intact integral membrane proteins in aqueous/organic solvent mixtures compatible with biological ‘soft-ionization’ mass spectrometry, preserving covalent structure into the gas phase. Multiply charged protein ions are fragmented in the gas phase, using either collision-activated or electron-capture dissociation, thus yielding complex spectra of sequence-dependent product ions that collectively define the original native covalent state of an intact protein. Top down offers a more detail-orientated approach to post-transcriptional and post-translational diversity allowing an enhanced insight beyond genomic translation, which has now extended into the bilayer proteome.

Keywords::

• CAD
• collision-activated dissociation
• ECD
• electron-capture dissociation
• electrospray-ionization
• intact mass tags
• intact proteins
• integral membrane proteins
• mass spectrometry
• post-transcriptional modifications
• post-translational modifications
• top-down proteomics

Acknowledgements

We thank Neil Kelleher, Fred McLafferty and Alan Marshall for their tireless support of the field of top-down mass spectrometry, the reviewers for their insightful comments on the manuscript and the federal agencies (National Institutes of Health, National Science Foundation and Department of Energy) for their financial support (JPW).