FIG. 6.

ER events facilitating ER-to-cytosol transport of polyomaviruses. Polyomaviruses traffic from the cell surface to the ER attached to the ganglioside receptor. Step 1. Viral particles are released from the receptor into the ER lumen. The PDI family member ERp57 isomerizes viral disulfide bonds, releasing a subset of the major capsid VP1. Step 2. Additional conformational changes are conferred to the viral particle by other ER chaperones including ERp29, PDI, BiP, and ERdj3. These reactions expose the minor capsid proteins VP2 and VP3, increasing the viral hydrophobicity. Step 3. The viral particle binds to the ER membrane and is ejected into the cytosol as an intact particle by unknown mechanisms.