Caveolar endocytosis of simian virus 40 reveals a new two-step vesicular-transport pathway to the ER

Simian virus 40 (SV40) is unusual among animal Viruses in that it enters ce lls through caveolae, and the internalized virus accumulates in a smooth en doplasmic reticulum (ER) compartment. Using video-enhanced, dual-colour, li ve fluorescence microscopy, we show the uptake of individual virus particle s in CV-1 cells. After associating with caveolae, SV40 leaves the plasma me mbrane in small, caveolin-1-containing vesicles. It then enters larger, per ipheral organelles with a non-acidic pH. Although rich in caveolin-1, these organelles do not contain markers for endosomes, lysosomes, ER or Golgi, n or do they acquire ligands of clathrin-coated vesicle endocytosis. After se veral hours in these organelles, SV40 is sorted into tubular, caveolin-free membrane vesicles that move rapidly along microtubules, and is deposited i n perinuclear, syntaxin 17-positive, smooth ER organelles. The microtubule- disrupting agent nocodazole inhibits formation and transport of these tubul ar carriers, and blocks viral infection. Our results demonstrate the existe nce of a two-step transport pathway from plasma-membrane caveolae, through an intermediate organelle (termed the caveosome), to the ER. This pathway b ypasses endosomes and the Golgi complex, and is part of the productive infe ctious route used by SV40.