Mutations within the transmembrane glycoprotein of Mason-Pfizer monkey virus: loss of SU-TM association and effects on infectivity.

We have previously reported that an 11-amino-acid deletion (D33) within the immunosuppressive peptide (ISP) region of the Mason-Pfizer monkey virus transmembrane (TM) glycoprotein, gp22, caused the loss of interaction between TM and the surface (SU) glycoprotein, gp70. This resulted in the secretion of large amounts of biologically active SU glycoprotein, and we postulated that the ISP might represent a point of contact between the two glycoproteins. To further define the amino acids that might be involved in this proposed region of interaction, we have made two neighboring 4-amino-acid deletions within the area defined by the D33 mutation and have carried out saturation mutagenesis on this 8-amino-acid region. We found that one of the smaller deletions (delta D), and two single point mutations (R68S and L72P), gave the same phenotype as the original D33 mutant. These results provide additional support for the hypothesis that this region of the TM glycoprotein is in contact with the SU glycoprotein and is important in maintaining the noncovalent interactions of the glycoproteins that function to hold the complex together.