Hard on the heels of the revelation that it’s the shape of the receptor that matters for H5N1 and other flu virus binding to cells, rather than the receptor chemistry, comes the finding reported in the J Virology of Feb 2008 that there is a distinct difference in the receptor binding ability of the supposed SARS coronavirus (SARSCoV) progenitor found in horseshoe bats, and SARSCoV itself – despite the viruses being very similar in genome organisation and indeed genome sequence.
SARSCoV isolates from humans bind angiotensin-converting enzyme 2 (ACE2) in order to gain entry into cells. While the virus is very closely related to isolates found in several Himalayan palm civets and a raccoon dog in Chinese live-meat markets, palm civets in particular outside markets were largely free from SARS-CoV infection – indicating to epidemiologists that this was not the natural animal reservoir of SARSCoV. Other studies determined that there was a group of CoVs very similar to SARSCoV in horseshoe bats – but that these viruses differed from human isolates mainly in the N-terminal regions of their S proteins, which enable virus entry into host cells. The SARS-like CoV (SL-CoV) S proteins have significant sequence divergence in the receptor-binding domain (RBD) from SARS-CoV S protein RBDs, including two deletions of 5 and 12 or 13 aa, and it has been predicted that SL-CoVs would not use ACE2 as a receptor.
In the absence of infectious isolates of SL-CoVs, these authors tested this hypothesis using an HIV-derived pseudovirus system: this used luciferase-expressing HIV-derived DNA constructs co-transfected into HeLa cells stably transduced with the ACE2 receptor gene from human, bat or civet, together with S gene-expressing plasmid constructs. The system results in HIV-like virions containing RNA which expresses luciferase, with S protein on their surfaces. Binding of pseudovirions to cells and their subsequent uptake was assayed by luminometry after addition of the luciferase substrate to cell lysates.
They found that that the bat SL-CoV S protein was unable to use ACE2 for cell entry regardless of the origin of the ACE2, and that the human SARS-CoV S could not use bat ACE2 as a functional receptor. Interestingly, after replacement of a small segment SLCoV S protein by the cognate sequence of SARSCoV S, the SL-CoV S protein also bound human ACE2.
The authors claim that this study reveals the “first example of host switching achievable for G2b CoVs [the taxonomic group including SL-CoVs] under laboratory conditions by the exchange of a relatively small sequence segment”. They speculate that, given that bats may be coinfected by several CoVs, and that they associate at very high density, and CoVs have a tendency towards recombination, that it is reasonable to assume that bats act as a natural mixing vessel for CoVs, which can result in the emergence of novel viruses which could easily cross species barriers.
Adding fuel to the speculative fire is another paper in the same issue: this reports that there is evidence of a recombinant origin for SL-CoVs, and there is probably “…an uncharacterized SLCoV lineage that is phylogenetically closer to S[ARS]CoVs than any of the currently sampled bat SLCoVs.”
So let’s all just wait for the next one, shall we?