Genus-specific pattern of intrinsically disordered central regions in the nucleocapsid protein of coronaviruses.

The nucleocapsid (N) protein is conserved in all four genera of the coronaviruses, namely alpha, beta, gamma, and delta, and is essential for genome functionality. Bioinformatic analysis of coronaviral N sequences revealed two intrinsically disordered regions (IDRs) at the center of the polypeptide. While both IDR structures were found in alpha, beta, and gamma-coronaviruses, the second IDR was absent in deltacoronaviruses. Two novel coronaviruses, currently placed in the genus, appeared intermediate in this regard, as the second IDR structure could be barely discerned with a low probability of disorder. Interestingly, these two are the only coronaviruses thus far isolated from marine mammals, namely beluga whale and bottlenose dolphin, two highly related species; the N proteins of the viruses were also virtually identical, differing by a single amino acid. These two unique viruses remain phylogenetic oddities, since gammacoronaviruses are generally avian (bird) in nature. Lastly, both IDRs, regardless of the coronavirus genus in which they occurred, were rich in Ser and Arg, in agreement with their disordered structure. It is postulated that the central IDRs make cardinal contributions in the multitasking role of the nucleocapsid protein, likely requiring structural plasticity, perhaps also impinging on coronavirus host tropism and cross-species transmission.