THE SEQUENCE 130-137 OF HUMAN INTERFERON-ALPHA-2 IS INVOLVED IN THE COMPETITION OF INTERFERON, PROTHYMOSIN-ALPHA AND CHOLERA-TOXIN-B SUBUNIT FOR COMMON RECEPTORS ON HUMAN FIBROBLASTS

I-125-labelled recombinant human interferon alpha 2 (rHuIFN-alpha 2) c apable of high-affinity binding (K-d = 2.46 +/- 0.18 x 10(-10) M) with receptors expressed on mouse thymocytes was obtained. Prothymosin alp ha (proTM-alpha) but not cholera toxin was found to compete with radio labelled IFN-alpha 2 for binding to the same receptor (K-i = 3.68 +/- 0.21 x 10(-11) M). The synthetic peptide covering the sequence 130-137 of IFN-alpha 2 (authors' definition: alpha- peptoferon) was shown to have the capacity to displace the labelled IFN-alpha 2 from the IFN-al pha 2/receptor complex (K-i = 7.19 +/- 0.12 x 10(-11) M). It was shown that receptors of this type are localized in plasmatic membrane fract ion. Using [I-125]-alpha-peptoferon, specific and saturable binding wa s detected on human fibroblasts and the data fitted a single binding s ite. Scatchard analysis yielded a K-d Of 9.63 +/- 0.17 x 10(-8) M. The binding was competitively inhibited by IFN-alpha 2 (the K-i value in competition assays was 1.37 +/- 0.12 x 10(-3) M), proTM-alpha (K-i = 2 .2 +/- 0.2 x 10(-7) M) and cholera toxin B subunit (K-i = 5.5 +/- 0.2 x 10(-7)). The present study has demonstrated that the sequence 130-13 7 of HuIFN-alpha 2 is involved in the competition of HuIFN-alpha 2, pr oTM-alpha and cholera toxin B subunit for common receptors on human fi broblasts.