While glycosylation is the most abundant protein modification, phosphory-lation is the most important posttranslational modification with regulatory function. Over 50% of all proteins are thought to be able to undergo phosphorylation, but at a given time point, only 2% of all proteins are estimated to be phosphorylated (Reinders and Sickmann 2005). Knowledge about protein phosphorylation sites and the conditions under which these sites are being used is crucial for our understanding of signaling networks and their dynamics. In mammalian tissue, acid hydrolysis and autoradiography has revealed that about 2 to 4% of the acid-stable phosphate is present in the form of phos-photyrosine; the remaining majority is in serine and threonine (Galski et al. 1983). Several tools and techniques for robust identification of phosphoryla-tion sites by mass spectrometry have been published recently, which also are readily applicable for proteins extracted from plant tissues (Beausoleil et al. 2004; Nuhse et al. 2003; Olsen and Mann 2004; Peck 2006).
Sulfatation is a rather common modification of tyrosine residues that has not been well studied to date. In mammalian tissue it has been estimated that about 1% of all tyrosine residues are sulfated (Bauerle and Huttner 1985). The discovery of tyrosylprotein sulfotransferase activities in plant microso-mal membrane fractions indicates that protein sulfatation also plays a role in a variety of plants (Hanai et al. 2000). However, the role of this modification for signaling processes needs to be further elucidated. Methylation and acetylation are known to have a regulatory function in the cell nucleus during growth, development, and under certain stress conditions (Butterbrodt et al. 2006; Tai et al. 2005; Tsuji et al. 2006; Xu et al. 2005). Other modifications, such as carboxylation and hydroxylation, are only now starting to move into our view. Their role in signaling processes is far less well understood, and is likely to yield plenty of novel and exciting discoveries in the near future.
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