Intracellular Turnover, Novel Secretion, and Mitogenically Active Intracellular Forms of v-sis Gene Product in Simian Sarcoma Virus-transformed Cells

In simian sarcoma virus (SSV)-transformed cells (SSV-NRK, SSV-NIH 3T3, and SSV-NPl cells), the vsis gene product was synthesized as a 36-kDa glycopolypeptide with one endoglycosidase (Endo) H-sensitive oligosaccharide chain and formed a dimer (~72) with a half-time of <5 min. p72 was proteolytically processed to generate sequentially ~68 and ~58 in the endoplasmic reticulum/Golgi complex, p44 in the postGolgi complex compartments, and p27 in an endosomal/lysosomal compartment. A portion (20-30%) of p72 and ~68 later became Endo H-resistant but Endo F-sensitive. During processing, the v-sis gene products exhibited rapid turnover, possibly in the endoplasmic reticulum and/or Golgi complex. The rate of turnover correlated with the tumorigenicity previously reported in these SSV-transformed cells. All three SSV-transformed cells secreted v-sis gene product (~44). p44 was secreted but remained tightly associated with the cell surface. This novel secretion provided an efficient system for the interaction of p44 with the cell surface platelet-derived growth factor receptor which resulted in the intracellular formation of ~27. A fraction of secreted p44 was converted extracellularly to a 27-kDa product (extracellular ~27) after a longer time in culture. The identical N-terminal amino acid sequence of p44 and extracellular p27 (H2N-SLGSLSVAEPAMIA) indicated a preferential site (Lys”’ -Argl”) for the proteolytic processing. The intracellular turnover of the v-sis gene product and its correlation with tumorigenicity as well as the demonstration of mitogenically active intracellular forms of v-sis gene product support the hypothesis of intracellular loop autocrine transformation.