Actually, I am now skeptical. The reasoning seems circular. The pangolin theory seems based upon the fact that although the bat virus RaTG13 is 96.12% identical to SARS-Cov2, the pangolin-CoV has the ACE2 site, and is 91.02% and 90.55% identical to SARS-CoV-2 and BatCoV RaTG13. https://www.ncbi.nlm.nih.gov/pubmed/32197085 And the above authors are saying that since the S1 in pangolin is closer to SARS-Cov2 it must have had a gain of ACE2 binding in humans. But what about the 96.12% identical RaTG13? Is it possible that someone was messing with RaTG13 and drove a mutation that resulted in ACE2-binding? And here is that backbone (MG772934 and MG772933) being collected and grown in rats and human cells in 2018: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6135831/ Also, rat ACE2, not crazy different from human ACE2. This from 2005. Ok, I am descending into #mkbatshittery. I need to stop.In this study, a suckling rat model was initially used to study the possibility of the proliferation of bat-derived CoVs in other animals. Previously, only one report had shown promising results associated with the isolation of live SL-CoVs from the fecal samples of bats with Vero E6 cells13. The live SL-CoV cultured in Vero E6 cells presented a typical CoV morphology and has the ability to use ACE2 from humans, civets, and Chinese horseshoe bats for cell entry33. An attempt to isolate the virus with Vero E6 cells was unsuccessful, which was likely due to a low viral load or a lack of compatibility with Vero E6 cells. This study found that the SL-CoVs derived from bats could replicate successfully in suckling rats, and pathological examination showed the occurrence of inflammatory reactions in the examined organs of the suckling rats. This result indicated that the virus can proliferate in rats and has the potential of cross-species transmission. When CoV particles procured from the infected brain tissues of the rats were studied by electron microscopy, the morphology of the particles was found to be identical to the typical coronavirus particles, as described in previous studies34. However, the typical spikes could not be visualized by electron microscopy. This observation can be partially explained by the hypothesis that the S1 and S2 domains of the S protein (which are not well-connected) were easily detached from the virion using excessive freeze-thawing or ultracentrifugation6.
By altering histidine 353 in rat ACE2 and modifying a glyosylation site that may alter the shape of a-helix 1, we converted rat ACE2 to an efficient receptor for SARS-CoV.