We have produced a cell line which lacks the protein tyrosine kinase JAK1 and is completely defective in interferon response. Complementation of this mutant with JAK1 restored the response, establishing the requirement for JAK1 in both the interferon-alpha/beta and -gamma signal transduction pathways. The reciprocal interdependence between JAK1 and Tyk2 activities in the interferon-alpha pathway, and between JAK1 and JAK2 in the interferon-gamma pathway, may reflect a requirement for these kinases in the correct assembly of interferon receptor complexes.

The protein-tyrosine kinases (PTKs) are a burgeoning family of proteins, each of which bears a conserved domain of 250 to 300 amino acids capable of phosphorylating substrate proteins on tyrosine residues. We recently exploited the existence of two highly conserved sequence elements within the catalytic domain to generate PTK-specific degenerate oligonucleotide primers (A. F. Wilks, Proc. Natl. Acad. Sci. USA 86: [1603][1604][1605][1606][1607] 1989). By application of the polymerase chain reaction, portions of the catalytic domains of several novel PTKs were amplified. We describe here the primary sequence of one of these new PTKs, JAK1 (from Janus kinase), a member of a new class of PTK characterized by the presence of a second phosphotransferase-related domain immediately N terminal to the PTK domain. The second phosphotransferase domain bears all the hallmarks of a protein kinase, although its structure differs significantly from that of the PTK and threonine/ serine kinase family members. A second member of this family (JAK2) has been partially characterized and exhibits a similar array of kinase-related domains. JAK1 is a large, widely expressed membrane-associated phosphoprotein of approximately 130,000 Da. The PTK activity of JAK1 has been located in the C-terminal PTK-like domain. The role of the second kinaselike domain is unknown.Protein-tyrosine kinases (PTKs) are structurally well suited to a role in intracellular signal transduction. Many growth factor receptors, for example, transduce the extracellular stimulus they receive through interaction with their cognate ligand via an intracellular tyrosine kinase domain (5, 33, 52; reviewed in reference 60). Members of the PTK family each bear a highly related "catalytic" domain. The phylogenetic relationships established by an amino acid sequence comparison of the catalytic domains (10) are borne out in the overall structure of the PTKs. For example, families of PTKs, such as those based on the structure of the colony-stimulating factor-1 growth factor receptor (38) (including the two types of the platelet-derived growth factor receptor [4,58]) and the protooncogene c-kit [59]) and those clustered around the cytoplasmic PTKs c-src (29) (including HCK/bmk [12], LCK [28], and c-yes [42], among others) and c-fes (37) (including c-FER/flk [11,25]) each share the highest degree of identity with other members of their cluster and, in respect to their overall topology, are structurally more related to each other than to members of other classes of PTK. Hence, the recombination of the PTK catalytic domain with a wide variety of regulatory and other interactive domains suggests a strong evolutionary drive toward the rapid expansion of the use of its physiologically powerful catalytic activity. This combinatorial approach to the evolution of multidomain proteins such as the PTIK family predicts the extensive utilization of the basic tyrosine kinase domain in other metabolic niches.Application of the polymerase chain reaction (PCR) (32, 40) using degenerate PTK-spec...

The mechanism of action of prolactin (PRL) was studied in murine lymphoid BAF‐3 cells transfected with either the long form of the PRL receptor (PRL‐R), or a chimeric receptor consisting of the extracellular domain of the PRL‐R and the transmembrane and intracellular domain of the erythropoietin receptor (PRL/EPO‐R). PRL sustained normal and long‐term proliferation of BAF‐3 cells expressing either the PRL‐R or the hybrid PRL/EPO‐R. Upon [125I]PRL cross‐linking, both types of BAF‐3 transfectants were shown to express two [125I]PRL cross‐linked species differing in size by 20 kDa. These cross‐linked complexes, after denaturation, were recognized by antibody against the PRL‐R, indicating that they contain the transfected receptor. PRL induced rapid and transient tyrosine phosphorylation of both the PRL‐R and the PRL/EPO‐R in BAF‐3 transfectants. Furthermore, PRL induced rapid tyrosine phosphorylation of the Janus kinase 2 (JAK2) which was already physically associated with the PRL‐R or the PRL/EPO‐R in the absence of ligand. JAK1 was also associated with PRL‐R and PRL/EPO‐R in the absence of ligand. However, only in BAF‐3 cells expressing the PRL‐R does PRL induce rapid and transient tyrosine phosphorylation of JAK1. These results demonstrate that JAK protein tyrosine kinases couple PRL binding to tyrosine phosphorylation and proliferation.