Jak2 is a nonreceptor tyrosine kinase that acts in numerous cellular signal transduction systems. Here, large amounts of recombinant Jak2 protein were expressed in eukaryotic cells, and an unknown 55 kDa protein was copurified with it. Mass spectrometry and Western blot analysis identified the 55 kDa protein as the alpha- and beta-isoforms of tubulin. Biochemical experiments determined that Jak2 and tubulin specifically coassociate with one another, and the region of Jak2 that binds tubulin is the pseudokinase domain. Immunofluoresence indicated that Jak2 and tubulin (microtubules) colocalize within intact cells. The functional consequence of the coassociation between Jak2 and tubulin is that Jak2 phosphorylates tubulin on tyrosine residues. Specifically, in response to growth hormone, tubulin was phosphorylated in a Jak2-dependent manner. Tubulin was also found to interact with signal transducers and activators of transcription 1 (STAT1) and be involved in STAT1 nuclear transport. As such, this work suggests that tubulin is a substrate of Jak2 and facilitates Jak2/STAT1-dependent signaling.