Cytokine receptors do not contain intrinsic kinase domains, thus the recruitment and activation of their downstream kinases, termed the Janus kinases (Jaks), is a highly regulated process. The Jaks consist of a tyrosine kinase and a pseudokinase domain in addition to several smaller conserved sequences located near the NH2-terminus. The domain located closest to the NH2-terminal is essential for the binding of Jaks to cytokine receptor subunits; however, the same region also appears to have some influence on the catalytic activity of the kinase domain. Now, Zhou et al. have found that the Jak3 NH2-terminus consists of a FERM (4.1, ezrin, radixin, moesin) domain that mediates intermolecular binding to the common γ (γc) cytokine receptor chain, and simultaneously stabilizes, through intramolecular interactions, the active conformation of the kinase domain of Jak3. Experiments in COS-7 cells expressing γc with various mutant forms of Jak3 revealed that mutations in the FERM domain disrupted the association of Jak3 with γc. In addition, patient-derived mutations in the FERM domain greatly reduced the kinase activity of Jak3; these mutants also failed to bind ATP. These data suggest that the intact FERM domain is required for protein-protein association and for properly maintaining the structure of the kinase domain. Experiments using COS cells that expressed the isolated kinase domain of Jak3 and various NH2-terminal fragments of Jak3 revealed that the FERM domain associated with the kinase domain, and that the FERM domain enhanced the kinase activity in vitro. Staurosporine, a kinase inhibitor that binds to the ATP binding site of kinases and alters the conformation of kinases, disrupted the association of Jak3 with γc, and this effect was reversed by the addition of excess amounts of ATP. Thus, these data suggest that the FERM domain of Jak3 simultaneously functions to promote association with cytokine receptors while mediating the appropriate activation of the kinase domain.
Y.-J. Zhou, M. Chen, N. A. Cusack, L. H. Kimmel, K. S. Magnuson, J. G. Boyd, W. Lin, J. L. Roberts, A. Lengi, R. H. Buckley, R. L. Geahlen, F. Candotti, M. Gadina, P. S. Changelian, J. J. O'Shea, Unexpected effects of FERM domain mutations on catalytic activity of Jak3: Structural implication for Janus kinases. Mol. Cell 8, 959-969 (2001). [Online Journal]