Sequencing intact membrane proteins using MALDI mass spectrometry

Membrane proteins are key players in many cellular events and represent crucial drug targets. Matrix-assisted laser desorption ionization mass spectrometry (MALDI MS) is a valuable approach to investigate them. To our knowledge, there are only a few reports of sequencing small membrane proteins using MALDI in-source decay (ISD). We report the successful fragmentation and sequencing of membrane proteins up to 46 kDa by MALDI-ISD. We have 1) investigated key MALDI parameters that influence the sequencing of a soluble protein; 2) used atomic force microscopy to observe our samples and correlate their topological features with MALDI data, which allowed us to optimize fragmentation conditions; 3) sequenced N- and C-termini of three membrane proteins (SpoIIIAF, TIM23, and NOX), solubilized in three different ways. Our results indicate that detergent and buffer type are of key importance for successful MALDI-ISD sequencing. Our findings are significant because sequencing membrane proteins enables the unique characterization of challenging biomolecules. The resulting fragmentation patterns provide key insights into the identity of proteins, their sequences, modifications, and other crucial information, such as the position of unexpected truncation.