Detection and characterization of a factor which rescues spliceosome assembly from a heat-inactivated HeLa cell nuclear extract.

Mild heat treatment of HeLa cell nuclear extracts (NE) selectively inhibits pre-mRNA splicing. Heat-inactivated extracts can be complemented by a small amount of untreated NE. Utilizing this complementation assay and a combination of ion-exchange, affinity, and hydrophobic chromatography, a heat reversal factor (HRF) was purified from NE that is required to rescue pre-mRNA splicing from a heat-inactivated extract. This activity in its most purified form consistently copurified in a fraction containing two 70-kDa proteins and a minor polypeptide of approximately 100 kDa. It was free of the major small nuclear RNAs, sensitive to protease, and required to rescue spliceosome formation from a heat-inactivated nuclear extract. These results suggest that this factor is a protein that may be an important component in pre-mRNA splicing, or alternatively, it may be involved in renaturation of a heat-sensitive splicing factor.