DIFFERENTIAL ACTIVATION OF ACUTE-PHASE RESPONSE FACTOR STAT3 AND STAT1 VIA THE CYTOPLASMIC DOMAIN OF THE INTERLEUKIN-6 SIGNAL TRANSDUCER GP130 .2. SRC HOMOLOGY SH2 DOMAINS DEFINE THE SPECIFICITY OF STAT FACTORACTIVATION/

Distinct yet overlapping sets of STAT transcription factors are activa ted by different cytokines. One example is the differential activation of acute phase response factor (APRF, also called Stat3) and Stat1 by interleukin 6 and interferon-gamma. Interleukin 6 activates both fact ors while, at least in human cells, interferon-gamma recruits only Sta t1. Stat1 activation by interferon-gamma is mediated through a cytosol ic tyrosine motif, Y440, of the interferon-gamma receptor. In an accom panying paper (Gerhartz, C., Heesel, B., Sasse, J., Hemmann, U., Landg raf, C., Schneider-Mergener, J., Horn, F., Heinrich, P. C., and Graeve , L. (1996) J. Biol. Chem. 271, 12991-12998), we demonstrated that two tyrosine motifs within the cytoplasmic part of the interleukin 6 sign al transducer gp130 specifically mediate APRF activation while two oth ers can recruit both APRF and Stat1. By expressing a series of Stat1/A PRF domain swap mutants in COS-7 cells, we now determined which domain s of Stat1 and APRF are involved in the specific recognition of phosph otyrosine motifs. Our data demonstrate that the SH2 domain is the sole determinant of specific STAT factor recruitment. Furthermore, the SH2 domain of Stat1 is able to recognize two unrelated types of phosphoty rosine motifs, one represented by the interferon-gamma receptor Y440DK PH peptide, and the other by two gp130 YXPQ motifs. By molecular model ing, we propose three dimensional model structures of the Stat1 and AP RF SH2 domains which allow us to explain the different binding prefere nces of these factors and to predict amino acids crucial for specific peptide recognition.