his team improved and extended a previously developed technology, which enabled them to identify for the first time all phosphorylations of all proteins in living cells - and in their temporal dynamic. For this approach cell cultures were stimulated by EGF for different lengths of time. The "Epidermal Growth Factor" is known for causing the phosphorylation of a multitude of enzymes and proteins along a signal transduction pathway. In the following step all proteins were isolated from the cells, divided into different fractions and analysed via mass spectrometry. This technology allows the precise identification of structure and composition of unknown compounds, here the cellular proteins. In total, 6,600 specific phosphorylation sites in 2,244 proteins were detected. "Comparing our results with the listings in existing databases showed that more than 90 percent of our sites were novel. This suggests that the majority of cellular phosphorylation sites still await identification."

Equally surprising was the discovery that about half of all cellular proteins harbour more than one phosphorylation sites, which in many cases behave differently. "This makes more than one way of phosphorylation possible where proteins serve as integrating platforms for a variety of incoming stimuli", says Mann. "This integration of signals could be independent, with phosphorylation of each site occurring separately from the others. It could also be dependent so that a 'priming site' has to be phosphorylated first for the subsequent modification of all other sites in the protein. In any case, the degree of phosphorylation should always be measured site specifically rather than for the protein as a whole". For the efficient use of their results the research team created the Phosida database (www.phosida.com), where all the phosphorylation sites are listed with additional information and connections to respective listings in other databases. An i
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Contact: Professor Matthias Mann
mmann@biochem.mpg.de
Max-Planck-Gesellschaft
10-Nov-2006

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