Summary Although tyrosine phosphorylation of extracellular proteins has been reported to occur extensively in vivo, no secreted protein tyrosine kinase has been identified. As a result, investigation of the potential role of extracellular tyrosine phosphorylation in physiological and pathological tissue regulation has not been possible. Here we show that VLK, a putative protein kinase previously shown to be essential in embryonic development, is a secreted protein kinase, with preference for tyrosine, that phosphorylates a broad range of secreted and ER-resident substrate proteins. We find that VLK is rapidly and quantitatively secreted from platelets in response to stimuli, and can tyrosine phosphorylate co-released proteins utilizing endogenous as well as exogenous ATP sources. We propose that discovery of VLK activity provides an explanation for the extensive and conserved pattern of extracellular tyrosine phosphophorylation seen in vivo, and extends the importance of regulated tyrosine phosphorylation into the extracellular environment.

In our experiments exploring VLK substrates, we carried out SILAC on both human C28/I2 cells and mouse MLB13MYC cells. However, the data reported in the paper are based on the mouse cell experiments, and we mistakenly indicated in the Extended Experimental Procedures that C28/I2 cells were used for the generation of these data. This has been corrected in the article online and does not affect any reported data or interpretations in the paper.Cell 159, 955, November 6, 2014 ª2014 Elsevier Inc. 955