Protein synthesis required to anchor a mutant p⁵³ protein which is temperature-sensitive for nuclear transport

Abstract

THE p⁵³ protein is rendered temperature-sensitive by a point mutation¹. Rat cells transformed by this mutant p⁵³ and an activated ras oncogene grow well at 37°C but cease DNA synthesis and cell division when shifted to 32°C (ref. 1). Immunostaining demonstrates that the mutant p⁵³ protein is in the nucleus of the arrested cells at 32°C but in the cytoplasm of the growing cells at 37°C. This is the first example of a protein which is temperature-sensitive for nuclear transport. The translocation from cytoplasm to nucleus and vice versa occurs 6 h after temperature shift and is coincident with the inhibition of DNA synthesis; transport from cytoplasm to nucleus does not require protein synthesis. Remarkably, inhibition of protein synthesis at 37°C also results in the rapid appearance of mutant p⁵³ in the cell nucleus. These results suggest the presence of a short-lived protein responsible for holding p⁵³ in the cytoplasm at 37°C but not at 32°C. Analysis of a non-temperature-sensitive mutant p⁵³ protein shows that its cytoplasmic location is sensitive to protein synthesis inhibitors but not to temperature.