Western blotting was conducted as previously described (Zeng et al., 2020 (link)). Briefly, cells were collected and lysed in 200 ul of RIPA buffer (50 mM Tris (pH 7.5), 150 mM NaCl, 1 mM EDTA, Nonidet P-40, 0.1% SDS) in the presence of protease inhibitor cocktail (MilliporeSigma, P8340), followed by clarification at 13200 rpm for 10 minutes, and boiling for 10 minutes at 100°C with 1x SDS loading buffer. To determine the Spike content in virion particles, pseudovirus supernatant was collected, filtered and purified by ultracentrifugation through a 20% sucrose cushion. The purified virions were dissolved in 1x SDS loading buffer. Subsequently, the samples were separated by 10% SDS-PAGE gels, transferred to PVDF membranes and immunoblotted with anti-S1 (Sino Biological, 40150-T62), anti-S2 (Sino Biological, 40590-T62), anit-GAPDH (Santa Cruz, sc-47724), and anti-p24 (anti-p24 (NIH ARP-1513) antibodies, followed by immunoblotting with anti-mouse-IgG-Peroxidase (Sigma, A5278) or anti-rabbit-IgG-HRP (Sigma, A9169) antibodies.