Given that larger proteins generally give rise to a greater number of peptides following digestion, and thus a greater number spectral counts, relative protein abundance is commonly standardized to account for protein size. Rappsilber et al. used “protein abundance index” (PAI), which represents the number

of peptides identified divided by the number of theoretically observed peptides, to quantify the relative abundance of proteins detected by MS analyses [61]. Zybailov et al. and Florens and Washburn used “normalized spectral abundance factor” (NSAF), which represents the number of spectral counts divided by protein length [62, 63]. In this study, we have quantified 2D-HPLC-MS/MS abundance profiles based on each proteins “relative abundance index” selleck products (RAI), calculated as the number of spectral counts (SpC) divided by molecular mass (Mr) of protein. While selleck chemicals llc the number of proteins detected by shotgun 2D-HPLC-MS/MS was greater than 4-plex 2D-HPLC-MS/MS, RAI values followed a similar trend, further verifying general protein abundance using both acquisition methods ( Additional file 1). However, the RIA per a given protein was lower using the 4-plex versus shotgun acquisition Proteases inhibitor method. This was expected given that the 4-plex run simultaneously measures four samples and

associated labels, thus reducing available peptide acquisition time. Due to the increased sensitivity and deeper coverage, we use the RAI data of shotgun exponential phase samples when discussing relative protein expression profiles in the text. Changes in stationary phase protein expression levels using iTRAQ 2D-HPLC-MS/MS Understanding cellular responses to pH change, end-product accumulation, and substrate limitation may aid in improving

strain growth through targeted deregulation of factors that limit growth and production of desired end-products. Comparison of expression levels of two biologically replicated iTRAQ-labelled exponential phase and stationary phase samples (tagged with reporter ions 114 & 115 and 116 & 117, respectively) was performed using 4-plex 2D-HPLC-MS/MS. Ratios of z-score values among exponential and stationary phase biological replicates (reporter ion ratios 115/114 vs 117/116) and between exponential phase vs stationary phase samples Cyclic nucleotide phosphodiesterase (reporter ion ratio 116/114 vs 117/115) are plotted in Additional file 2a and 2b, respectively, to illustrate correlation between biological replicates. While Additional file 2a shows good correlation between biological replicates (perfect correlation represented by coordinates 0,0), a number of proteins have poorer correlation between replicates. To determine the statistical significance of protein expression ratios between exponential and stationary phase samples when factoring in the deviation between biological replicates, z-scores ratios for each protein were converted into vectors, and the vector difference was calculated (see Methods).