STRUCTURAL ELEMENTS IN GLYCOPROTEIN-70 FROM POLYTROPIC FRIEND MINK CELL FOCUS-INDUCING VIRUS AND GLYCOPROTEIN-71 FROM ECOTROPIC FRIEND MURINE LEUKEMIA-VIRUS, AS DEFINED BY DISULFIDE-BONDING PATTERN AND LIMITEDPROTEOLYSIS

The disulfide-bonding pattern of glycoprotein 70 (gp70), the surface g lycoprotein (SU) encoded by the envelope gene of polytropic Friend min k cell focus-inducing virus, was elucidated and compared with that of glycoprotein 71 (gp71), the corresponding glycoprotein of the ecotropi c Friend murine leukemia virus, which had previously been determined ( M. Linder, D. Linder, J. Hahnen, H.-H. Schott, and S. Stirm, Eur. J. B iochem. 203:65-73, 1992). In the carboxy-terminal constant domain, in which these glycoproteins have about 97% sequence homology, the locati on of the four disulfide bonds was found to be analogous. In the amino -terminal differential domain, with about 37% sequence homology, 8 of the 12 cysteine residues of the ecotropic SU are conserved in the poly tropic SU. In this domain, a similar clustering of disulfide bonds was detected, which led to the identification of three distinct disulfide -bonded regions in both glycoproteins. However, because of deletions a nd sequence deviations, the glycoproteins must have significantly diff erent three-dimensional structures in these regions. Since the recepto r-binding functions of both glycoproteins have been attributed to thei r amino-terminal domains and since each binds to a different receptor, these disulfide-bonded structures are likely candidates for receptor- binding functions. Limited proteolysis of both glycoproteins with vari ous endoproteinases led to the identification of preferential proteoly tic sites between disulfide-bonded regions, at the beginning of the hy pervariable proline-rich region, and between differential and constant domains, further confirming the structural organization of the folded glycoproteins.