Coronavirus Cell Entry Occurs through the Endo-/Lysosomal Pathway in a Proteolysis-Dependent Manner
Figure 7
Introduction of a furin cleavage site just upstream of the fusion peptide renders MHV independent of lysosomal proteases.
A) Schematic representation of the MHV spike protein. The MHV S proteins are partially processed by furin at the S1/S2 boundary (S1/S2) as indicated by the arrow. The furin cleavage site sequence at this position (RRAHR) is shown. The signal sequence (SS) at the amino-terminal end of the S1 subunit and the approximate positions of the fusion peptide (FP), heptad repeat regions 1 and 2 (HR1 and HR2) and the transmembrane domain (TM) in the S2 subunit are indicated. MHV-S2′FCS virus contains an optimal furin cleavage site (RRRRR) immediately upstream of the FP (S2′, indicated by the arrow. B) Effect of pan-lysosomal protease inhibitor (CPI) on MHV and MHV-S2′FCS infection. HeLa-mCC1a cells were pretreated with CPI for 30 min and inoculated at MOI = 0.2 with luciferase expression cassette containing MHV-EFLM or MHV-S2′FCS in the presence of CPI, after which incubations were continued in the presence of CPI until 7 hpi. Infection levels were determined by measuring the luciferase activity in cell lysates relative to mock-treated cells. Error bars represent SEM, n = 3*3.