Grape berry plasma membrane proteome analysis and its differential expression during ripening

High purity berry plasma membranes (PMs) of Vitis vinifera L. cv. Cabernet Sauvignon were isolated by two-phase partitioning of microsome fractions at differ- ent stages of berry ripening. PM proteins resolvable by the detergent cocktail of CHAPS and ASB-14 were separated by two-dimensional electrophoresis. A total of 119 protein spots from pre-veraison berry PMs on 2- D gels detected with silver staining were subjected to MALDI-TOF mass spectrometry analysis. Sixty-two spots were identified as putative PM proteins, with 1-6 predicted transmembrane helices, including true PM proteins such as ATP synthase, ABC transporters, and GTP-binding proteins reported in plants. They were then grouped into eight functional categories, mainly involved in transport, metabolism, signal transduction, and protein synthesis. Another 11 spots were identified as proteins of unknown function. The veraison and post-veraison samples stained 98 and 86 spots on the gels, respectively. During the berry ripening process, total PM protein content gradually decreased. Among all identified proteins, 12 showed significant differ- ences in terms of their relative abundance. Increasing ubiquitin proteolysis and cytoskeleton proteins were observed from pre-veraison to post-veraison. Zeatin O- glucosyltransferase peaked at veraison, while ubiquitin- conjugating enzyme E2-21 was down-regulated at this stage. This proteome research provides the first in- formation on PM protein characterization during the grape berry ripening process.