Lamp1 Increases the Efficiency of Lassa Virus Infection by Promoting Fusion in Less Acidic Endosomal Compartments.

Christine E Hulseberg,Katarzyna M Szymańska,Lucie Fénéant,Judith M White,Anne Moscona

doi:10.1128/mbio.01818-17

Abstract

ABSTRACTLassa virus (LASV) is an arenavirus whose entry into host cells is mediated by a glycoprotein complex (GPC) comprised of a receptor binding subunit, GP1, a fusogenic transmembrane subunit, GP2, and a stable signal peptide. After receptor-mediated internalization, arenaviruses converge in the endocytic pathway, where they are thought to undergo low-pH-triggered, GPC-mediated fusion with a late endosome membrane. A unique feature of LASV entry is a pH-dependent switch from a primary cell surface receptor (α-dystroglycan) to an endosomal receptor, lysosomal-associated membrane protein (Lamp1). Despite evidence that the interaction between LASV GP1 and Lamp1 is critical, the function of Lamp1 in promoting LASV infection remains poorly characterized. Here we used wild-type (WT) and Lamp1 knockout (KO) cells to show that Lamp1 increases the efficiency of, but is not absolutely required for, LASV entry and infection. We then used cell-cell and pseudovirus-cell surface fusion assays to demonstrate that LASV GPC-mediated fusion occurs at a significantly higher pH when Lamp1 is present compared to when Lamp1 is missing. Correspondingly, we found that LASV entry occurs through less acidic endosomes in WT (Lamp1-positive) versus Lamp1 KO cells. We propose that, by elevating the pH threshold for fusion, Lamp1 allows LASV particles to exit the endocytic pathway before they encounter an increasingly acidic and harsh proteolytic environment, which could inactivate a significant percentage of incoming viruses. In this manner Lamp1 increases the overall efficiency of LASV entry and infection.

Highlights

Lassa virus (LASV) is an arenavirus whose entry into host cells is mediated by a glycoprotein complex (GPC) comprised of a receptor binding subunit, GP1, a fusogenic transmembrane subunit, GP2, and a stable signal peptide
We propose that Lamp1 increases the overall efficiency of LASV entry and infection by promoting fusion in a more hospitable, less acidic endosomal compartment
Our findings can be summarized as follows. (i) A robust (~85%) decrease in Lamp1 expression does not dampen the efficiency of LASV pseudovirus infection of 293T cells over a range of input multiplicities. (ii) Knockout of Lamp1 expression in 293T cells diminishes, but does not abolish, entry and infection. (iii) LASV GPC-mediated mbio.asm.org 7

Summary

Introduction

Lassa virus (LASV) is an arenavirus whose entry into host cells is mediated by a glycoprotein complex (GPC) comprised of a receptor binding subunit, GP1, a fusogenic transmembrane subunit, GP2, and a stable signal peptide. By elevating the pH threshold for fusion, Lamp allows LASV particles to exit the endocytic pathway before they encounter an increasingly acidic and harsh proteolytic environment, which could inactivate a significant percentage of incoming viruses. In this manner Lamp increases the overall efficiency of LASV entry and infection. The acidified environment within maturing endosomes eventually triggers fusion between the viral and endosomal membranes, allowing the viral genome to be released through the resulting fusion pore into the cytoplasm This membrane fusion event is mediated by the viral glycoprotein complex (GPC), which is comprised of a receptor binding subunit, GP1, a fusogenic transmembrane subunit, GP2, and a stable signal peptide. As the pH of the endocytic compartment decreases, GP1 dissociates from the complex, triggering major reorganizational changes in GP2 and uncovering the hydrophobic fusion loop that drives membrane fusion [8,9,10]

Methods

Results

Conclusion