Janus kinase (Jak) subcellular localization revisited: The exclusive membrane localization of endogenous Janus kinase 1 by cytokine receptor interaction uncovers the Jak·receptor complex to be equivalent to a receptor tyrosine kinase

The Janus kinases are considered to be cytoplasmic kinases that constitutively associate with the cytoplasmic region of cytokine receptors, and the Janus kinases (Jaks) are crucial for cytokine signal transduction. We investigated Jak1 localization using subcellular fractionation techniques and fluorescence microscopy (immunofluorescence and yellow fluorescent protein-tagged Jaks). In the different experimental approaches we found Jak1 (as well as Jak2 and Tyk2) predominantly located at membranes. In contrast to previous reports we did not observe Jak proteins in significant amounts within the nucleus or in the cytoplasm. The cytoplasmic localization observed for the Jak1 mutant L80A/Y81A, which is unable to associate with cytokine receptors, indicates that Jak1 does not have a strong intrinsic membrane binding potential and that only receptor binding is crucial for the membrane recruitment. Finally we show that Jak1 remains a membrane-localized protein after cytokine stimulation. These data strongly support the hypothesis that cytokine receptor-Janus kinase complexes can be regarded as receptor tyrosine kinases.