PTM MAPPING IN HUMAN H-RAS UNDER OXIDATIVE STRESSES

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This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. H-Ras is a plasma membrane associated GTPase, which is an important player in signal transduction to control cell proliferation, differentiation, and invasion. There are four reactive cysteines (C118, C181, C184 and C186) on H-Ras. These cysteines are targets of posttranslational modifications (PTM) which may alter the cellular localization and function of this protein. In this study, PTMs on H-Ras in human aortic endothelial cells (HAECs) is being investigated by mass spectrometry to obtain the full map of H-Ras modifications under different oxidative stresses. H-Ras is not an abundant protein that can be easily extracted from cell lines. A protocol has been developed to facilitate the enrichment and purification of the H-Ras, where exogenous H-Ras with 6X His tag was transduced by adenovirus, and pulled down by magnetic Ni-NTA beads. Tryptic in-gel digestion was performed on the His-tagged H-Ras, followed by MS analysis using both MALDI-TOF and LTQ-Orbitrap. Although high sequence coverage was obtained by combining these two methods, only C181 and C184 containing peptides were identified in the mass spectrum. The C186 containing tryptic peptide CVLS was absent in either experiment, because VLS is known to be cleaved after C186 gets farnesylated. Since two cysteine residues of interest were not covered in the bottom-up analysis, a top-down approach by Fourier-transform ion cyclotron resonance (FT-ICR) MS is now being developed. Several methods have been tried for the purification of the H-Ras sample, including those that use the Ziptip (C4 and C18) or centrifugal filters, dialysis, and the CapLC. It was found that the combination of dialysis and use of the Poros 50 R1 column provides the best result with the highest protein recovery yield and least interference from polyethylene glycol (PEG) and other undesirable contaminants. HPLC and Michrom desalting trap will be tried next.