Truncation of the nuclear localization signal of polyomavirus VP1 results in a loss of DNA packaging when expressed in the baculovirus system

Using the pBlueBacIII baculovirus transfer vector, N11-VP1, a truncated for m of the polyomavirus major capsid protein VP1, was cloned for expression i n the baculovirus-insect cell expression system. The N11-VP1 protein is vir tually identical to full-length, wild-type VP1, except that the first 11 am ino acids have been deleted from the amino terminus of the protein. The N-t erminal region of VP1 has previously been shown to contain the nuclear loca lization signal (NLS) of the protein and contains residues essential for bo th nuclear transport as well as DNA-binding functions. The 5-day infected S f9 cellular lysate from the recombinant N11-VP1 preparation was purified by cesium chloride density gradient centrifugation. Capsid-like particles wer e observed in the resulting preparation. The purified particle preparation was analyzed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis a s well as Western blotting and was shown to have accurately expressed the N 11-VP1 as cloned. Examination of the Coomassie-stained gels revealed that t he capsid-like particles composed of the N11-VP1 protein did not contain an y host-derived histones. The absence of the histones in the N11-VP1 capsid- like particles is indicative of the inability of these particles to package DNA, a feature which is observed when wild-type VP1 is treated in this man ner. Electron microscopy of these particles substantiated this observation. To determine if the deletion of the NLS exhibited true in vivo characteris tics, Sf9 insect cells were infected with the recombinant baculovirus carry ing the N11-VP1 gene and examined early in infection (30 h post-infection) by indirect immunofluorescence. The N11-VP1 protein was not transported to the nucleus and remained in the cytoplasm. When the Sf9 cells were coinfect ed with N11-VP1 and polyomavirus VP2 and VP3 carrying baculoviruses, the N1 1-VP1 was transported to the nucleus by cooperation with the minor capsid p roteins. These studies demonstrate that the N-terminal region of VP1, which contains the NLS and DNA-binding domains, is essential for VP1 nuclear tra nsport and its ability to package Sf9 cellular DNA. (C) 1998 Elsevier Scien ce B.V. All rights reserved.