The J domain of papovaviral large tumor antigen is required for synergistic interaction with the POU-domain protein Tst-1/Oct6/SCIP

Large T antigens from polyomaviruses are multifunctional proteins with role s in transcriptional regulation, viral DNA replication, and cellular transf ormation. They have been shown to enhance the activity of various cellular transcription factors. In the case of the POU protein Tst-1/Oct6/SCIP, this enhancement involves a direct physical interaction between the POU domain of the transcription factor and the amino terminal region of large T antige n. Here we have analyzed the structural requirements for synergistic intera ction between the two proteins in greater detail. Tst-1/Oct6/SCIP and the r elated POU protein Brn-1 were both capable of direct physical interaction w ith large T antigen. Nevertheless, only Tst-1/Oct6/SCIP functioned synergis tically with large T antigen. This differential behavior was due to differe nces in the amino-terminal regions of the proteins, as evident from chimera s between Tst-1/Oct6/SCIP and Brn-1. Synergy was specifically observed for constructs containing the amino-terminal region of Tst-1/Oct6/SCIP, Large T antigen, on the other hand, functioned synergistically with Tst-1/Oct6/SCI P only when the integrity of its J-domain-containing amino terminus was mai ntained. Mutations that disrupted the J domain concomitantly abolished the ability to enhance the function of Tst-1/Oct6/SCIP, The J domain of T antig en was also responsible for the physical interaction with Tst-1/Oct6/SCIP a nd could be replaced in this property by other J domains. Intriguingly, a h eterologous J domain from a human DnaJ protein partially substituted for th e amino terminus of T antigen even,vith regard to the synergistic enhanceme nt of Tst-1/Oct6/SCIP function. Given the general role of J domains, we pro pose chaperone activity as the underlying mechanism for synergy between Tst -1/Oct6/SCIP and large T antigens.