The Drosophila Dpit47 protein is a nuclear Hsp90 co-chaperone that interacts with DNA polymerase alpha

Hsp90 is gaining increasing importance as a protein involved in controlling the normal functioning of the cell. To do this it apparently interacts wit h a battery of co-chaperone proteins that are involved in both substrate re cognition and the progression of the Hsp90 catalytic pathway. In this repor t we have identified the Drosophila Dpit47 protein (DNA polymerase interact ing tpr containing protein of 47 kDa) through its interaction with the DNA polymerase a. This protein is a predominantly nuclear protein, which forms a tight and stoichiometric interaction with Hsp90 and shows interaction wit h Hsp70. It also has substantial homology to other known Hsp90 co-chaperone s, e.g. CNS1 and hop1, making it likely that this protein also functions as an Hsp90 co-chaperone. The interaction with the DNA polymerase a is not related to the special sit uation in early embryos where there are large amounts of maternal protein s tockpiles of the polymerase, as it occurs to the same level in early and la te embryos and also in proliferating cell culture. However it does not occu r in quiescent cells, making it likely that the protein is related to proli feration. This is also consistent with Dpit47 expression being higher in pr oliferating cells. The interaction between the Dpit47 and the polymerase ta kes place predominantly in the nucleoplasm, and seems to involve several su bunits of the polymerase in comparable amounts, making it unlikely that it is solely required for the assembly of the polymerase complex. The polymera se can also be seen to interact with Hsp90, and the interaction between Dpi t47 and the polymerase is increased by the specific Hsp90 inhibitor geldana mycin. This suggests that a complex of the Dpit47, Hsp90 and DNA polymerase exists in the cell. The interaction between DNA polymerase alpha and Dpit4 7 completely inhibits the activity of the polymerase. These results suggest that Hsp90 acts as a chaperone for DNA polymerase cc and that this interaction is mediated through the novel co-chaperone Dpit47 . This provides the first suggestion of a role for chaperones in DNA replic ation in higher eukaryotes.