Clathrin- and caveolin-1–independent endocytosis

TLDR

SV40 normally reaches the endoplasmic reticulum via caveolar endocytosis. In caveolin‑1–deficient cells and wild‑type fibroblasts, SV40 exploits a cav‑1‑independent endocytic route. The virus is rapidly internalized in cholesterol‑ and tyrosine‑kinase–dependent, clathrin‑free vesicles that traffic along microtubules to pH‑neutral organelles and the ER, demonstrating a clathrin‑ and caveolae‑independent pathway that remains active even when cav‑1 is present.

Abstract

Simian Virus 40 (SV40) has been shown to enter host cells by caveolar endocytosis followed by transport via caveosomes to the endoplasmic reticulum (ER). Using a caveolin-1 (cav-1)–deficient cell line (human hepatoma 7) and embryonic fibroblasts from a cav-1 knockout mouse, we found that in the absence of caveolae, but also in wild-type embryonic fibroblasts, the virus exploits an alternative, cav-1–independent pathway. Internalization was rapid (t1/2 = 20 min) and cholesterol and tyrosine kinase dependent but independent of clathrin, dynamin II, and ARF6. The viruses were internalized in small, tight-fitting vesicles and transported to membrane-bounded, pH-neutral organelles similar to caveosomes but devoid of cav-1 and -2. The viruses were next transferred by microtubule-dependent vesicular transport to the ER, a step that was required for infectivity. Our results revealed the existence of a virus-activated endocytic pathway from the plasma membrane to the ER that involves neither clathrin nor caveolae and that can be activated also in the presence of cav-1.