Caveolae are involved in the trafficking of mouse polyomavirus virions and artificial VP1 pseudocapsids toward cell nuclei

Electron and confocal microscopy were used to observe the entry and the movement of polyomavirus virions and artificial virus-like particles (VP1 pseudocapsids) in mouse fibroblasts and epithelial cells. No visible differences in adsorption and internalization of virions and VP1 pseudocapsids (empty or containing DNA) were observed. Viral particles entered cells internalized in smooth monopinocytic vesicles, often in the proximity of larger, caveola-like invaginations. Both empty vesicles derived from caveolae and vesicles containing viral particles were stained with the anti-caveolin-1 antibody, and the two types of vesicles often fused in the cytoplasm. Colocalization of VP1 with caveolin-1 was observed during viral particle movement from the plasma membrane throughout the cytoplasm to the perinuclear area. Empty vesicles and vesicles with viral particles moved predominantly along microfilaments. Particle movement was accompanied by transient disorganization of actin stress fibers. Microfilaments decorated by the VP1 immunofluorescent signal could be seen as concentric curves, apparently along membrane structures that probably represent endoplasmic reticulum. Colocalization of VP1 with tubulin was mostly observed in areas close to the cell nuclei and on mitotic tubulin structures. By 3 h postinfection, a strong signal of the VP1 (but no viral particles) had accumulated in the proximity of nuclei, around the outer nuclear membrane. However, the vast majority of VP1 pseudocapsids did not enter the nuclei.