MALDI in-source decay, from sequencing to imaging.
In-source decay (ISD) fragmentation as combined with matrix-assisted laser desorption/ionization (MALDI) mass spectrometry allows protein sequencing directly from mass spectra. Acquisition of MALDI-ISD mass spectra from tissue samples is achieved using an appropriate MALDI matrix, such as 1,5-diaminonaphthalene (DAN). Recent efforts have focused on combining MALDI-ISD with mass spectrometry imaging (MSI) to provide simultaneous sequencing and localization of proteins over a thin tissue surface. Successfully coupling these approaches requires the development of new data analysis tools, but first, investigating the properties of MALDI-ISD as applied to mixtures of protein standards reveals a high sensitivity to the relative protein ionization efficiency. This finding translates to the protein mixtures found in tissues and is used to inform the development of an analytical pipeline for data analysis in MALDI-ISD MS imaging, including software to identify the most pertinent spectra, to sequence protein mixtures, and to generate ion images for comparison with tissue morphology. The ability to simultaneously identify and localize proteins is demonstrated by using the analytical pipeline on three tissue sections from porcine eye lens, resulting in localizations for crystallins and cytochrome c. The variety of protein identifications provided by MALDI-ISD-MSI between tissue sections creates a discovery tool, and the analytical pipeline makes this process more efficient.