Entry Pathways of an Avian Virus into Cells Expressing Transmembrane and GPI-Anchored Receptor Isoforms

Abstract

Avian Sarcoma and Leukosis Virus (ASLV) entry into cells proceeds through two major steps -priming by cognate receptors on the cell surface followed by low pH-induced fusion with endosomes. Subtype A viruses are able to utilize two naturally occurring isoforms of the TVA receptor for infection, the GPI-anchored (TVA800) and the transmembrane (TVA950) receptors, which are localized to different membrane domains. We monitored the consecutive steps of ASLV entry into cells expressing these receptor isoforms, including endocytosis, delivery into acidic compartments, and fusion with endosomes. Cells expressing TVA950 internalized ASLV much faster than those expressing the alternative receptor. In both cell lines, fusion occurred shortly after internalization, suggesting that virus uptake was the rate-limiting step of entry. We found that ASLV fusion with endosomes proceeded through two relatively long-lived intermediates, a hemifusion-like intermediate and a small fusion pore. The lifetime of these intermediates was dependant on TVA receptor isoforms and their expression levels. TVA950 supported more robust fusion intermediates compared to TVA800. These findings are consistent with ASLV trafficking by via TVA950 to endosomal compartments that are more conducive to fusion. Alternatively, transmembrane receptor might more efficiently prime the viral fusion proteins compared to the GPI-anchored receptor. This work has been supported by the NIH AI053668 grant.