Protein abundance statistics
After testing, the number of proteins identified in Gayals samples were G1(4956 pcs), G2(4972 pcs), G3(4474 pcs) and in Yellow Cattle samples were Y1 (4754 pcs), Y2 (5425 pcs), Y3 (5382 pcs).
Analysis of rumen microbial protein abundance
Rumen bacteria
At the level of rumen bacteria phyla, 20 phyla were detected in Gayals and 21 phyla in Yellow Cattle, more than 95% of the gene sets came from 5 phyla. The abundance of Firmicutes and Actinobacteria in Gayals was significantly higher than that in Yellow Cattle (
P<0.05), but the abundances of Bacteroidetes, Proteobacteria and Chloroflexi were significantly lower than those of Yellow Cattle (
P<0.05) (Table 1).
At the level of rumen bacteria genus, 209 and 224 genera were detected in Gayal and Yellow Cattle, respectively, among which 15 genera had protein abundance percentages more than 1%. Difference analysis showed that the abundances of
Olsenella and
Ruminococcus in Gayals were significantly higher than those in Yellow Cattle (
P<0.05), while the abundances of
Prevotella and
bacteroides in Gayals were significantly lower than those in Yellow Cattle (
P<0.05) (Table 2).
At the phyla level, Firmicutes, Bacteroidetes, Actinobacteria and Proteobacteria were the dominant bacteria in Gayals and Yellow Cattle. Many studies have shown that Firmicutes and Bacteroidetes play a leading role in rumen fiber degradation (
Stevenson, 2007;
Leng, 2011). There are a high number of fiber degrading bacteria in rumen and Firmicutes is the main phylum for fiber degradation, which contains a large number of cellulose degrading bacteria (
Matsui, 2000;
Krause, 2003).
Rumen fungi
At the level of rumen fungal phyla, 6 phyla were detected in Gayals and Yellow Cattle,
viz. Ascomycota, Neocallimas tigomycota, Chytridiomycota, Basidiomycota, Glomeromycota and Cryptomycota, respectively. The abundance of Chytridiomycota in Gayals was significantly lower than that in Yellow Cattle (
P<0.05) (Table 3).
At the level of rumen fungal genus, 25 genera were detected in Gayals and Yellow Cattle. Among them, 23 genera with protein abundance percentage greater than 1% were
Batrachochytrium,
Rhizophagus,
Piromyces,
Absidia,
Mortierella,
Neocallimastix,
Rhizopus,
Lichtheimia,
Chaetomium,
Fomitiporia,
Myceliophthora, Rozella,
Schizosaccharomyces,
Brettanomyces,
Coprinopsis,
Heterobasidion,
Malassezia,
Millerozyma,
Mucor, Pestalotiopsis,
Rhizoctonia,
Saccharomyces,
Tremella,
Candida and
Kluyveromyces.
Batrachochytrium and
Rhizophagus had the highest protein abundance percentage and the abundance of
Batrachochytrium in Gayals was significantly lower than that in Yellow Cattle (
P<0.05) (Table 4).
At the phyla level, only 6 phyla were detected and the abundance of Chytridiomycota and Batrachochytrium in Gayals were significantly lower than that in Yellow Cattle (
P<0.05). Although the content of rumen fungi is less, anaerobic fungal hyphae can penetrate the plant cell wall and dissolve lignin and produce a series of cellulose degrading enzymes with high activity, such as cellulase, hemicellulase and esterase, which can be assembled into a complex with high catalytic activity (
Akin, 1990). Studies have shown that there are six major genera involved in lignocellulose degradation in the rumen anaerobic fungi:
Neocallimastix,
Piromyces,
Caecomyces,
Anaeromyces,
Orpinomyces and
Cyllamyces. Among them, the lignocellulose degradation efficiency of
Neocallimastix and
Piromyces was higher (
Puniya, 2015). In this experiment, the protein abundance of
Neocallimastix and
Piromyces in Gayals was more than 1%, which belonged to the main genus detected by rumen fungi and which was of great significance to the high cellulose degradation activity of Gayals. Among them,
Ruminococcus flavus and
Ruminococcus alba are the important fibrous decomposing bacteria in the rumen (
Henderson, 2015).
Ruminococcus was the dominant genus of rumen bacteria in Gayals and Yellow Cattle, which was consistent with the previous research results and the protein abundance of
Ruminococcus in Gayals was significantly higher than that in Yellow Cattle (
P<0.05), which may be related to the strong fiber degradation ability of Gayals.
Differentially expressed proteins
The results showed that there were 561 up-regulated proteins (the protein expression of Yellow Cattle was higher than that of Gayals) and 172 down-regulated proteins (the protein expression of Gayals was higher than that of Yellow Cattle) in the 733 differentially expressed proteins, only 27 proteins were identified in Gayals and 33 proteins were identified in Yellow Cattle (Table 5). Firmicutes and Bacteroidetes were the most abundant among up-regulated proteins and down-regulated proteins. Among the up-regulated proteins, 6.42% were more than 5-fold difference. There were three proteins with more than 10-fold difference, which were from norank (the differential protein was 23.23-fold),
Bacteroides sp.4_1_36 in Bacteroides and
Tannerella Forsythia ExBB protein (K03561) (the differential protein was 11.36-fold). Among the down regulated proteins, there were three proteins with a differential multiple of more than 10 fold and the biggest differential multiple was
Bacteroides sp CAG:770-the differential protein PYG (k00688) (16.26 fold), followed by two norank differential proteins (16.21 and 11.13 fold, respectively). Fig 1 showed the difference of rumen microbial proteins between Gayals and Yellow Cattle. Each point in the figure represents a specific protein, it can be seen from the figure that the number of up-regulated proteins is more than that of down-regulated proteins.
Function of rumen microbial protein
Functional annotation of differentially expressed protein KEGG
Total 733 differentially expressed proteins were annotated by KEGG and 142 metabolic pathways were found. The most active metabolic pathway was Glycolysis/Gluconeogenesis pathway. There were 14 metabolic pathways with the number of differential proteins more than 1% and 128 metabolic pathways with less than 1%, more than 5% of the metabolic pathways were metabolic pathways (11.78%), of which 11.42% were down-regulated (protein expression in Gayals was higher than that of Yellow Cattle), metabolism of microorganisms in different environments (8.42%), down-regulated protein accounted for 18.05%, carbon metabolism effect (7.10%), expression of down-regulated protein accounted for 15.29%, antibiotic biosynthesis (7.27%) and down-regulated protein was 16.38%, the metabolic synthesis of secondary organisms (6.44%), down-regulated protein expression accounted for 20.38%, Glycolysis/gluconeogenesis (5.58%), down-regulated protein accounted for 16.91%. In the KEGG functional annotation, a large number of sequences were involved in carbohydrate metabolism and the main metabolic pathways include starch and sucrose metabolism, glycolysis/gluconeogenesis metabolism and pyruvate metabolism,
etc. Among the identified differentiated protein metabolism pathways, most of the differentiated proteins came from the dominant phyla of rumen bacteria-Firmicutes, Bacteroidetes, Actinobacteria and Proteobacteria. The abundance of β-glucosidase of Clostridium in Gayals is higher than that in Yellow Cattle, as one of the important enzymes of fiber decomposition, β-glucosidase can completely hydrolyze cellobiose to glucose, rumen ferments glucose to maintain its own growth and finally ferments into volatile fatty acids, which can be absorbed by rumen wall to provide energy for ruminants.
Carbohydrate metabolism
Starch and sucrose metabolic pathway
In the pathway of starch sucrose metabolism (Fig 2), the difference protein quantity of Firmicutes was more than Bacteroides in the rumen bacteria of Gayals and Yellow Cattle and the most detected enzymes were phospho glucomutase (K01835, 5.4.2.2) and glucose-1-phosphate acyltransferase (K00975, 2.7.7.27). Glucoso phosphate protease was mainly from Firmicutes and partly from Actinobacteria and Cyanobacteria, the enzyme species with significant difference (
P<0.01) in the down-regulation of protein expression (the protein expression of Gayals was higher than that of Yellow Cattle) were
Atopobium sp. oral taxon 199,
Atopobium rimae,
Atopobium sp. BS2,
Selenomonas ruminantium,
Olsenella sp. oral taxon 809,
Atopobium parvulum,
Olsenella uli,
Bacteroides sp. CAG:714, the differences were significant (
P<0.05) of enzymatic species were
Clostridium sp. KNHs209,
Fischerella sp. PCC 9431,
Clostridium sp. CAG:307,
Collinsella tanakaei,
Olsenella uli. Among them, the abundance of β-glucosidase (K05349, 3.2.1.21) produced by
Clostridium sp.KNHS209 from Firmicutes of Gayals was significantly higher than that of Yellow Cattle (
P<0.05). In the diagram of Starch and sucrose metabolic pathway,
sucrose-specific IIB component, β-glucosidase (3.2.1.21), glucose-1-phosphate acyltransferase (2.7.7.27), glycogen phosphorylase (2.4.1.1), phosphoglucomutase (5.4.2.2) and glucose-6-phosphate isomerase (5.3.1.9) were down-regulated.
Glycolytic/gluconeogenic pathway
In the glycolysis/gluconeogenesis pathway (Fig 3), the differential proteins mainly come from Bacteroides and Firmicutes. A variety of enzymes were noted in the rumen glycolysis/gluconeogenesis process of Gayals and Yellow Cattle and it was found that glyceraldehyde-3-phosphate dehydrogenase (1.2.1.12) was a relatively abundant protein involved in this biological process in the rumen of Gayals. This enzyme was involved in the first step of the glycolytic pathway, catalyzing the reversible oxidative phosphorylation of D-glyceraldehyde-3-phosphate to 1, 3-bisphosphate-D-glycerol in the presence of NAD+ and phosphate and the process was most commonly facilitated by Firmicutes. Pyruvate feroxidase and phosphopyruvate kinase (4.1.1.32) were also detected, mainly from Bacteroides and Firmicutes, while the rest were from Fibrobacteria, Chlorobacteria, Spirochetes, Ignavibacteriae and Actinobacteria. Phosphofructokinase (PFK) was the rate limiting enzyme in the third step of glycolysis, which came from Firmicutes, the expression of down-regulated in
Selenomonas bovis and up-regulated in Firmicutes
bacterium CAG:227,
Lachnospiraceae bacterium AC2028 and
Blautia sp. CAG:257, Ruminococcus sp. CAG: 17. In down-regulated expression, alcohol dehydrogenase (1.1.1.1) produced by Bautia sp. Cag: 237,
Atopobium sp. BS2,
Eggerthia catenaformis, phosphoglucomutase (5.4.2.2) produced by
Clostridium sp. Cag: 307 and triosephosphate isomerase (5.3.1.1) produced by
Atopobium rimae,
Collinsella Tanakaei,
Oscillibacter sp.1-3, glyceraldehyde-3-phosphate dehydrogenase (1.2.1.12) produced by
Eggerthia Catenaformis. Phosphofructokinase (PFK) is the third rate limiting enzyme of glycolysis which comes from lunamonas ruminant and the expression of PFK in Gayals rumen is higher than that in Yellow Cattle. Compared with Yellow Cattle, the abundance and species sources of glyceraldehyde-3-phosphate dehydrogenase and phosphofructokinase in the rumen of Gayals were less, the abundance of
Olsenella profusa 6-phosphate-β-glucosidase in Gayals was significantly higher than that in Yellow Cattle (
P<0.05) in the abundance analysis, it is speculated that 6-phosphate-β-glucosidase is one of the key enzymes in the degradation of crude fiber in Gayals, 6-phosphate-β-glucosidase can catalyze 6-phosphate-β-glucoside compounds such as 6-phosphate-cellobiose and 6-phosphate-cellulose oligosaccharide to generate glucose-6-phosphate and decompose cellulose (Thompson, 1999; Desai, 2010). In the metabolic pathway, 6-phospho-β-glucosidase decomposes Arbutin-6P and Salicin-6P into β-D-glucose for subsequent catabolism. (2-3)-diphosphoglyceride phosphoglycerate mutase can catalyze the conversion of Glycerate-3P to Glycerate-2P, which is an important enzyme in gluconeogenesis and glycolysis. Previous studies detected the digestive protein products in rumen fluid of dairy cows fed high concentrate dairy cows by two-dimensional polyacrylamide gel electrophoresis (2DSDS-PAGE), indicating that many prokaryotic proteomes contained enzymes involved in glycolysis, such as glyceraldehyde-3-phosphate dehydrogenase, phosphoenolpyruvate carboxylic kinase(PEPCK), phosphoglycerate kinase and triosephosphate isomerase (Snelling, 2017).
Butanoate metabolic pathway
In the butanoate metabolic pathway (Fig 4), the number of differential proteins Firmicutes was more than Bacteroides between Gayals and Yellow Cattle, the most identified protein was Pyruvate feroxidase (K03737), the down-regulated enzymes were acetolactate synthase (2.2.1.6) and butyryl CoA dehydrogenase (1.3.8.1) and the acetolactate synthase was from Fusobacteria, acyl CoA dehydrogenase was mainly from Firmicutes and partly from Bacteroidetes. There are two production pathways of butyric acid, namely the direct conversion of butyric acid to acetic acid and the malonyl-CoA pathway, butyric acid can promote the development of rumen epithelium (
Sakata, 1978).
Pyruvate metabolic pathway
In the pathway of pyruvate metabolism (Fig 5), the number of differential proteins in Firmicutes was more than Bacteroides between Gayals and Yellow Cattle and the most identified protein was pyruvate feroxidase (K03737), mainly from Firmicutes and Bacteroides,
Kandleria vitulina,
Acidaminococcus sp.CAG:917,
Coprococcus catus and [Ruminococcus] Obeum of Firmicutes,
Coriobacteriaceae bacterium BV3Ac1 and
Atobicium fossor of Actinomycetes, then
Prevotella sp. CAG: 755 of Bacteroidetes in the rumen of Gayals were higher than those of Yellow Cattle. The down-regulated enzymes in metabolic pathway were pyruvate phosphate dikinase (4.1.1.49), D-lactate dehydrogenase (1.1.1.28), phosphotransacetylase (2.3.1.8) and pyruvate CoA transferase (2.8.3.1). There are three pathways for acetic acid formation, namely, direct conversion of pyruvate to acetic acid, the acetyl-CoA pathway and acetyl-phosphate pathway (
Söllinger, 2018). According to the pyruvate metabolic pathway diagram, the acetyl-CoA pathway and acetyl phosphate pathway are annotated in Gayals and Yellow Cattle, the acetyl-CoA synthesis pathway was reversible when the protein expression of Yellow Cattle was higher than that of Gayals, but the pathway was unidirectional when the protein expression of Gayals was higher than that of Yellow Cattle and the acetyl-CoA hydrolase was from
Phascolarctobacterium succinatutens. It was previously found that
Phascolarctobacterium wakonense sp. nov could produce both pyruvic acid and acetic acid when supplemented with pyruvate indicating that
Phascolarctobacterium plays an important role in rumen microbial acetic acid synthesis and acetylphosphate pathway is distributed in both up-regulation and down-regulation of protein expression (
Shigeno, 2019). Studies have shown that when feeding a large number of roughages, ruminants mainly obtain propionic acid through succinic acid decarboxylation pathway (
Oba, 2003), but there are few enzymes annotated by succinic acid pathway. D-lactate dehydrogenase and propionate-CoA transferase were annotated in the lactic acid-acrylic acid pathway and the expression of propionate-CoA transferase was down-regulated, D-lactate dehydrogenase came from Firmicutes and was only quantified in Gayals.