Jak2 acts as both a STAT1 kinase and as a molecular bridge linking STAT1 to the angiotensin II AT(1) receptor

Angiotensin II activates the Jak-STAT pathway via the AT(1) receptor. We st udied two mutant AT(1) receptors, termed M5 and M6, that contain Y to F sub stitutions for the tyrosine residues naturally found in the third intracell ular loop and the carboxyl terminus. After binding ligand, both the M5 and M6 AT(1) receptors trigger STAT1 tyrosine phosphorylation equivalent to tha t observed with the wild type receptor, indicating that angiotensin II-medi ated phosphorylation of STAT1 is independent of these receptor tyrosine res idues. In response to angiotensin II, Jak2 autophosphorylates on tyrosine, and Jak2 and STAT1 physically associate, a process that depends on the SH2 domain of STAT1 in vitro. Evaluation of the wild type, M5, and M6 AT(1) rec eptors showed that angiotensin II-dependent AT(1) receptor-Jak2-STAT1 compl ex formation is dependent on catalytically active Jak2, not on the receptor tyrosine residues in the third intracellular loop and carboxyl tail. Immun odepletion of Jak2 virtually eliminated the ligand-dependent binding of STA T1 to the AT(1) receptor. These data indicate that the association of STAT1 with the AT(1) receptor is not strictly bimolecular; it requires Jak2 as b oth a STAT1 kinase and as a molecular bridge linking STAT1 to the AT(1) rec eptor.