Human major HSP70 protein complements the localization and functional defects of cytoplasmic mutant SV40 T antigen in Swiss 3T3 mouse fibroblast cells.

The CT3 cytoplasmic localization mutant of SV40 T antigen is neither properly transported to the nucleus nor is it functional in rodent cells. Human precrisis cells are able to complement this mutation, as they are fully transformed by CT3 with wild-type efficiency. The human-specific factors responsible for this species-specific difference in response to CT3 were localized to human chromosome 6 by synteny in a panel of six somatic cell hybrids. A major human HSP70 heat shock protein located on chromosome 6 is expressed constitutively in human cells. Hsp70 proteins have been reported to play a role in intracellular movement of newly synthesized proteins. To test whether human HSP70 played a role in the complementation by human cells of the defect of CT3, we constructed a series of mouse cell lines expressing human HSP70 and tested them for their ability to localize CT3 T antigen in the nucleus and for their ability to be transformed by CT3 DNA. Mouse cell lines expressing human HSP70 protein were able to translocate mutant CT3 T antigen into the nucleus and were transformed by CT3 at rates comparable with wild-type SV40. Mouse-inducible HSP70 protein was not able to translocate cytoplasmic T antigen in Swiss 3T3 mouse fibroblast cells, even after heat shock. Apparently human HSP70 is capable of complementing directly or indirectly the structural and functional alterations in SV40 T antigen introduced by the CT3 mutation.