Sequence retrieval and alignments.
Full polyprotein 1ab (ORF1ab), spike (S) protein, membrane (M) protein, envelope (E) protein, and nucleocapsid (N) protein sequences were obtained for each of 34 distinct but representative alpha and betacoronaviruses from broad genus and subgenus distributions, including all known human coronaviruses (i.e., SARS-CoV, SARS-CoV-2, MERS-CoV, HKU1, OC43, NL63, and 229E). FASTA-formatted protein sequence data (the full accession number list is available in Table S5 in the supplemental material) were retrieved from the National Center of Biotechnology Information (NCBI) (67). For each of the protein classes (i.e., ORF1ab, S, M, E, and N), all 34 coronavirus sequences were aligned using the Clustal Omega v1.2.4 multisequence aligner tool employing the following parameters: sequence type [Protein], output alignment format [clustal_num], dealign [false], mBed-like clustering guide-tree [true], mBed-like clustering iteration [true], number of combined iterations 0, maximum guide tree iterations [-1], and maximum HMM iterations [-1] (68). For the purposes of estimating time of viral peptide production, we classified ORF1a and ORF1b peptides as “early” whereas all other peptides produced by subgenomic mRNAs were classified as “late” (69, 70).