Kinetics of nuclear transport and oligomerization of simian virus 40 large T antigen.

The kinetics of nuclear transport and of oligomerization of simian virus 40 (SV40) large T antigen in lytically infected cells were investigated by pulse-chase experiments, cell fractionation, and sedimentation analyses in sucrose gradients. After synthesis, large T was rapidly translocated to the nucleus. Within 10 min, half of the pulse-labeled molecules had entered the nucleus and after an additional 30 min, nuclear accumulation of large T reached a constant plateau of about 95%. Within that time, the majority of large T was in monomeric form suggesting that nuclear transport takes place in this state. In the nucleus, conversion to tetramers proceeded slowly and steadily. By 60 min half of the molecules had formed tetramers and by 6 hr a steady-state ratio between tetramers and monomers of 4:1 was observed. A small fraction of large T remaining in the cytoplasm oligomerized considerably faster than large T in the nuclear fraction. This phenomenon of accelerated oligomerization was also observed with a mutant of large T defective for nuclear transport. Perhaps, the nuclear envelope is a barrier for the complex forms of large T which prevents premature oligomers in the cytoplasm from entering the nucleus and oligomers in the nucleus from migrating back to the cytoplasm.