3.6  COG Function Classification of the DEPs
To further characterize the DEPs, COG function classification was also applied to categorize the proteins. As shown in Figure 4, the DEPs could be further classified into 18 categories (Supplementary File S4). Among them, 9 proteins were related to general function prediction only; 8 proteins were involved in transcription; 6 proteins were relevant to signal transduction mechanisms; 5 proteins were associated with protein turnover, posttranslational modification, and chaperones; 4 proteins were linked to vesicular transport, intracellular trafficking, and secretion; 4 proteins were correlated with RNA processing and modification; 3 proteins were correlated with chromatin structure and dynamics; 3 proteins were associated with carbohydrate transport and metabolism. Seven proteins were respectively related to one of the following biological functions: nucleotide transport and metabolism; lipid transport and metabolism; translation, ribosomal structure, and biogenesis; cell wall/membrane/envelope biogenesis; inorganic ion transport and metabolism; extracellular structures; and cytoskeleton. Notably, another 7 proteins related to unknown function were also identified (Figure 4). Therefore, further investigation concentrating on the function of these cellular proteins is certainly worth trying in the future.
Figure 4  COG function classification of the 78 differentially expressed proteins identified in IPEC-J2 cells upon PDCoV infection. The capital letters in abscissa denote the COG categories as marked on the right of the histogram and the ordinate indicates the number of proteins in each category.