Shotgun Proteomics of BN-PAGE-Resolved Chloroplasts

Blue Native-PAGE resolves protein complexes in their native state. When coupled with immunoblotting, it can be used to identify the presence of high molecular weight complexes at high resolution for any protein, given a suitable antibody. To identify proteins in high molecular weight complexes on a large-scale and to bypass the requirement for specific antibodies, we applied a tandem mass spectrometry (MS/MS) approach to BN-PAGE-resolved chloroplasts. Fractionation of wild-type chloroplasts into 6 bands allowed identification and label-free quantification of 1000 chloroplast proteins with native molecular weight resolution. Significantly, our approach achieves a depth of identification comparable to traditional shotgun proteomic analyses of chloroplasts, indicating much of the known chloroplast proteome is amenable to our ‘mass western’ approach to BN-PAGE. The coupling of BN-PAGE to MS/MS allows for a large-scale comparison of protein complexes between samples and the identification of proteins in high molecular weight complexes. In parallel we have analyzed chloroplasts from a leaf reticulate mutant (re-6) to demonstrate the possibility for comparative analyses. Our results provide a useful resource for the chloroplast community and this strategy is anticipated to be widely adaptable to other sub-cellular compartments.