Detection of G1 proteins in Chinese hamster cells synchronized by isoleucine deprivation or mitotic selection.
Examination of labeling patterns of proteins in Chinese hamster cells(line CHO) revealed the presence of a class of protein(s) that is synthesized during G1 phase of the cell cycle.  Cells arrested in G1 by isoleucine (Ile) deprivation were prelabeded with [14-C]Ile, induced to traverse G1 by addition of unlabeled Ile, and labeled with [3-H]Ile at hourly intervals.  Cells were fractionated into neclear and cytoplasmic portions, and proteins were separated by sodium dodecyl sulfate-polyacrylamide get electrophoresis.  Gel profiles of proteins in the 45,000-160,000 mol wt range from the cytoplasm of cells in G1 were similar to those from cells arrested in G1 except for the presence of a mojor peak of [1-H]Ile incorporated into a protein(s) of approximately 80,000 mol wt. Peaks of net [3-H]Ile incorporation were not detected in neclear preparations.  Cellular fractionation by differential centrifugation showed the peak I protein was located in the soluble supernatant fraction of the cytoplasm.  Time-course studies showed that synthesis of this protein began 1-2 h after initiation of G1 traverse; the protein reached maximum levels in 4-6 h and was reduced to undetectable levels by 9 h.  A cytoplasmic protein with similar electrophoretic mobility was found in G1 phase of cells synchronized by mitotic selection.  This class of proteins is synthesized by cells before entry into S phase and may be involved in initiation of DNA synthesis.
D000818:Animals; D002250:Carbon Radioisotopes; D002455:Cell Division; D002458:Cell Fractionation; D002460:Cell Line; D002467:Cell Nucleus / Q000378:metabolism; D002499:Centrifugation, Density Gradient; D006224:Cricetinae; D003593:Cytoplasm / Q000378:metabolism; D004591:Electrophoresis, Polyacrylamide Gel; D005260:Female; D007532:Isoleucine / Q000378:metabolism*; D008938:Mitosis; D008970:Molecular Weight; D010053:Ovary; D014176:Protein Biosynthesis; D013997:Time Factors; D014316:Tritium
D002250:Carbon Radioisotopes; D007532:Isoleucine; D014316:Tritium