Monitoring Protein Expression in Whole Bacterial Cells with MALDI Time-of-Flight Mass Spectrometry

We report the application of matrix-assisted laser desorption ionization (MALDI) to monitor recombinant protein expression in whole bacteria. This technique is characterized by rapid sample preparation that provides analysis of samples extracted directly from the growth media in less than 10 min. The mass spectrometric method holds several advantages over gel electrophoresis, the conventional method for examining the protein content of cells. Comparisons between the two methods of analysis are presented in terms of increased speed, efficiency, resolution, and mass accuracy. Delayed extraction time-of-flight mass spectrometry identifies posttranslational modifications and other changes in the expected structure which are not recognized by gel electrophoresis. The utility of this method is demonstrated for proteins with molecular masses ranging from 5 to 50 kDa. Low molecular mass proteins (10 kDa) can be efficiently analyzed without any treatment of the bacterial broth prior to MALDI sample preparation. The MALDI analysis of higher molecular weight proteins shows enhanced sensitivity when the bacterial solutions are first sonicated.