Mutisite phosphorylation of individual proteins appears to be quite common, and may be more the rule than the exception. While mapping multiple phospho- acceptor sites is now fairly reliable, understanding which phosphorylation sites modulate protein function or are active in a given biological pathway is still a difficult problem. Adding to the complexity of this problem is the fact that phosphorylation-dependent function may not depend on activity at a single site, but rather be dependent upon serial activation of several sites. We are developing and using mass spectrometry-based tools to provide a quantitative view of phosphorylation dependent structure-function relationships. With or without stable isotope tagging we are able to define a subset of the total in vivo phosphorylation sites which are functionally significant. Following this, one or multiple, specific epitopes can be monitored in a highly sensitive assay using mass spectrometry.
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