A mechanism for tyrosine phosphorylation of extracellular matrix proteins

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Summary A great number of proteins in the extracellular matrix are phosphorylated; yet practically all known kinases operate inside cells and are unable to catalyze phosphorylation of secreted proteins. The recent discovery of two secreted kinases, Fam20C and VLK, represents a major step forward in understanding how proteins outside cells can be phosphorylated at multiple sites. However, numerous extracellular proteins are phosphorylated at sites that are clearly not targets of these two kinases. Thus, additional secreted kinases must exist, raising questions about how to best identify them and determine what their functions may be. In this application the investigators describe how a bioinformatics strategy led to the discovery of secreted variants of transmembrane-spanning, ligand-activated, fibroblast growth factor receptor tyrosine kinases (FGFRs); with the variants consisting of the fibroblast growth factor-binding domain directly connected to the kinase domain. Based on preliminary data, showing that the secreted Fgfr2 kinase variant is expressed in multiple tissues in mice, the investigators have made several constructs for expression of the kinase in cultured cells and they have generated mice that can be induced to express the kinase in different cells and tissues. In Aim 1, in vitro and in vivo studies are directed at determining the specificity of the secreted FGFR2 kinase in phosphorylation of extracellular matrix proteins in skin, lung and craniofacial (skull) tissues. In Aim 2 experiments are designed to provide insights into the roles of the secreted FGFR2 kinase in developing lungs of mice and in three- dimensional organ cultures of lung tissues, allowing studies of cellular and molecular mechanisms that may be regulated by the secreted FGFR2 kinase. If successful, these studies are likely to break new ground and lead to future studies opening an entirely new chapter in the understanding of how cells interact with and regulate the properties of extracellular matrix molecules.