Protein Profiling with Cleavable Isotope-coded Affinity Tag (cICAT) Reagents

Protein expression profiles in yeast cells, in response to salinity stress, were determined using the cleavable isotope-coded affinity tag (cICAT) labeling strategy. The analysis included separation of the mixed protein samples by SDS-PAGE, followed by excision of the entire gel lane, and division of the lane into 14 gel regions. Regions were subjected to in-gel digestion, biotin affinity chromatography, and analysis by nano-scale microcapillary liquid chromatography coupled to tandem mass spectrometry. The novel 13C-labeled ICAT reagents have identical elution profiles for labeled peptide pairs and broadly spread the distribution of labeled peptides during reversed-phase chromatography. A total of 560 proteins were identified and quantified, with 51 displaying more than 2-fold expression differences. In addition to some known proteins involved in salt stress, four RNA-binding proteins were found to be up-regulated by high salinity, suggesting that selective RNA export from the nucleus is important for the salt-stress response. Some proteins involved in amino acid synthesis, which have been observed to be up-regulated by amino acid starvation, were also found to increase their abundance on salt stress. These results indicate that salt stress and amino acid starvation cause overlapping cellular responses and are likely to be physiologically linked.