A temperature-sensitive mutant of the mammalian RNA helicase, DEAD-BOX X isoform, DBX, defective in the transition from G1 to S phase

ts ET24 cells are a novel temperature-sensitive (ts) mutant for cell proliferation of hamster BHK21 cells. The human genomic DNA which rescued the temperature-sensitive lethality of ts ET24 cells was isolated and screened for an open reading frame in the deposited human genomic library. X chromosomal DBX gene encoding the RNA helicase, DEAD-BOX X isoform, which is homologous to yeast Ded1p, was found to be defective in this mutant. The single point mutation (P267S) was localized between the Motifs I and Ia of the hamster DBX of ts ET24 cells. At the nonpermissive temperature of 39.5°C, ts ET24 cells were arrested in the G1-phase and survived for more than 3 days. In ts ET24 cells, total protein synthesis was not reduced at 39.5°C for 24 h, while mRNA accumulated in the nucleus after incubation at 39.5°C for 17 h. The amount of cyclin A mRNA decreased in ts ET24 cells within 4 h after the temperature shift to 39.5°C, consistent with the fact that the entry into the S-phase was delayed by the temperature shift.