ABSTRACT
The reported identification of some peptide growth factors in the nucleus has prompted speculation that there may be an additional pathway involving transport to the nucleus following receptor-mediated endocytosis. Perhaps the most important issue concerning the regulation of cell growth by external ligands, such as peptide growth factors and hormones, remains the mechanism of signal transduction whereby the ligand stimulates various biological effects. Platelet-derived growth factor (PDGF) was originally purified as a small cationic protein which exhibited stability to heat and acid, but was rapidly inactivated by exposure to reducing agents. The inability to quantitatively remove all unbound ligand during the fractionation leaves open the possibility to artifactual nuclear contamination. The experiments described above provide experimental evidence for the physical localization of PDGF or related proteins in the nucleus of cells. The v-sis gene encodes an N-terminal signal sequence which results in the cotranslational translocation of the nascent polypeptide chain across the membrane of the endoplasmic reticulum.
