A Raft-derived, Pak1-regulated Entry Participates in α2β1 Integrin-dependent Sorting to Caveosomes

Abstract

We have previously shown that a human picornavirus echovirus 1 (EV1) is transported to caveosomes during 2 h together with its receptor α2β1 integrin. Here, we show that the majority of early uptake does not occur through caveolae. α2β1 integrin, clustered by antibodies or by EV1 binding, is initially internalized from lipid rafts into tubulovesicular structures. These vesicles accumulate fluid-phase markers but do not initially colocalize with caveolin-1 or internalized simian virus 40 (SV40). Furthermore, the internalized endosomes do not contain glycosylphosphatidylinositol (GPI)-anchored proteins or flotillin 1, suggesting that clustered α2β1 integrin does not enter the GPI-anchored protein enriched endosomal compartment or flotillin pathways, respectively. Endosomes mature further into larger multivesicular bodies between 15 min to 2 h and concomitantly recruit caveolin-1 or SV40 inside. Cell entry is regulated by p21-activated kinase (Pak)1, Rac1, phosphatidylinositol 3-kinase, phospholipase C, and actin but not by dynamin 2 in SAOS-α2β1 cells. An amiloride analog, 5-(N-ethyl-N-isopropanyl) amiloride, blocks infection, causes integrin accumulation in early tubulovesicular structures, and prevents their structural maturation into multivesicular structures. Our results together suggest that α2β1 integrin clustering defines its own entry pathway that is Pak1 dependent but clathrin and caveolin independent and that is able to sort cargo to caveosomes.