Independent polypeptide chain initiation sites for the synthesis of different classes of proteins for an RNA tumor virus: mouse mammary tumor virus.

To distinguish the synthesis of mouse mammary tumor virus (MMTV) structural proteins on a common precursor polypeptide from their independent synthesis on separate ribosome initiation sites, we have employed the NaCl hypertonic shock technique which differentially inhibits polypeptide chain initiation on different classes of messenger RNA in eukaryotic cells. Using this method to inhibit polypeptide initiation in the MMTV-producing C3H mouse cell line $\text{Mm5mt}\text{c}{}_1$, we have selectively inhibited the synthesis of MMTV glycosylated proteins (gp52 and gp36) relative to MMTV nonglycosylated proteins (p28, p22, p18, p14, p10). These results support the concept of independent polypeptide chain initiation sites for the synthesis of these two classes of MMTV proteins. Furthermore, the use of this technique provides evidence for the lack of strict coordinate synthesis of these proteins in an MMTV-infected cell.