Nucleotide sequence and expression of the spike (S) gene of canine coronavirus and comparison with the S proteins of feline and porcine coronaviruses

We have cloned, sequenced and expressed the spike (S) gene of canine coronavirus (CCV; strain K378). Its deduced amino acid sequence has revealed features in common with other coronavirus S proteins: a stretch of hydrophobic amino acids at the amino terminus (the putative signal sequence), another hydrophobic region at the carboxy terminus (the membrane anchor), heptad repeats preceding the anchor, and a cysteine-rich region located just downstream from it. Like other representatives of the same antigenic cluster (CCV-Insavc-1 strain, feline infectious peritonitis and enteric coronaviruses, porcine transmissible gastroenteritis and respiratory coronaviruses, and the human coronavirus HCV 229E), the CCV S polypeptide lacks a proteolytic cleavage site present in many other coronavirus S proteins. Pairwise comparisons of the S amino acid sequences within the antigenic cluster demonstrated that the two CCV strains (K378 and Insavc-1) are 93.3% identical, about as similar to each other as they are to the two feline coronaviruses. The porcine sequences are clearly more divergent mainly due to the large differences in the amino-terminal (residues 1 to 300) domains of the proteins; when only the carboxy-terminal parts (residues 301 and on) are considered the homologies between the canine, feline and porcine S polypeptides are generally quite high, with identities ranging from 90.8% to 96.8% . The human coronavirus is less related to the other members of the antigenic group. A phylogenetic tree constructed on the basis of the S sequences showed that the two CCVs are evolutionarily more related to the feline than to the porcine viruses. Expression of the CCV S gene using the vaccinia virus T7 RNA polymerase system yielded a protein of the expected M(r) (approximately 200K) which could be immunoprecipitated with an anti-feline infectious peritonitis virus polyclonal serum and which was indistinguishable from the S protein synthesized in CCV-infected cells.