Role of TSPAN9 in Alphavirus Entry and Early Endosomes.

UNLABELLED:Alphaviruses are small enveloped RNA viruses that infect cells via clathrin-mediated endocytosis and low-pH-triggered fusion in the early endosome. Using a small interfering RNA (siRNA) screen in human cells, we previously identified as a host factor that promotes infection by the alphaviruses Sindbis virus (SINV), Semliki Forest virus (SFV), and chikungunya virus (CHIKV). Depletion of Tduanyu1842N9 specifically decreases SFV membrane fusion in endosomes. Tduanyu1842N9 is a member of the tetraspanin family of multipass membrane proteins, but its cellular function is currently unknown. Here we used U-2 OS cells stably overexpressing Tduanyu1842N9 to show that Tduanyu1842N9 is localized at the plasma membrane and in early and late endosomes. Internalized SFV particles colocalized with Tduanyu1842N9 in vesicles early during infection. Depletion of Tduanyu1842N9 led to reductions in the amounts of the late endosomal proteins LAMP1 and CD63 and an increase in the amount of LAMP2. However, Tduanyu1842N9 depletion did not alter the delivery of SFV to early endosomes or change their pH or protease activity. Comparative studies showed that Tduanyu1842N9 depletion strongly inhibited infection by several viruses that fuse in early endosomes (SFV, SINV, CHIKV, and vesicular stomatitis virus [VSV]), while viruses that fuse in the late endosome (recombinant VSV-Lassa and VSV-Junin), including an SFV point mutant with a lower pH threshold for fusion (SFV E2 T12I), were relatively resistant. Our data suggest that Tduanyu1842N9 modulates the early endosome compartment to make it more permissive for membrane fusion of early-penetrating viruses. IMPORTANCE:Alphaviruses are spread by mosquitoes and can cause serious human diseases such as arthritis and encephalitis. Recent outbreaks of CHIKV infection are responsible for millions of cases of acute illness and long-term complications. There are no vaccines or antiviral treatments for these important human pathogens. Alphaviruses infect host cells by utilizing the endocytic machinery of the cell and fusing their membrane with that of the endosome. Although the mechanism of virus-membrane fusion is well studied, we still know relatively little about the host cell proteins that are involved in alphavirus entry. Here we characterized the role of the host membrane protein Tduanyu1842N9 in alphavirus infection. Tduanyu1842N9 was localized to early endosomes containing internalized alphavirus, and depletion of Tduanyu1842N9 inhibited virus fusion with the early endosome membrane. In contrast, infection of viruses that enter through the late endosome was relatively resistant to Tduanyu1842N9 depletion, suggesting an important role for Tduanyu1842N9 in the early endosome.

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