Roles of the HSP70-Subunit in a Eukaryotic Multi-Site-Specific Endonuclease, Endo.SceI: Autophosphorylation and Heat Stability

Abstract

The 70 kDa heat shock proteins (HSP70) are a family of molecular chaperones that bind transiently to unfolded proteins in an ATP/ADP dependent manner. Endo.SceI comprises a unique example for mitochondrial HSP70, which exists in a stable complex with a nucleolytic subunit as a multi-site specific DNase. The HSP70-subunit in Endo.SceI was autophosphorylated by ATP in vitro. The autophosphorylation was higher in the Endo.SceI complex form than in the free form. Although the autophosphorylation had no significant effect on the endonucleolytic activity of Endo.SceI, the factors favoring autophosphorylation protected the endonucleolytic activity of Endo.SceI against heat inactivation. ATP, adenosine 5′-O-(3-thiotriphosphate) (ATP-γ-S), and ADP not only protected the endonucleolytic activity against heat inactivation in the presence of Ca²⁺ ions, but also reduced the labeling of the HSP70-subunit by [γ-³²P]ATP in Endo.SceI. These findings suggest that the HSP70-subunit shields Endo.SceI from heat inactivation through ATP/ADP binding.