Abstract
Protein secretion systems are extremely important in bacteria because they are involved in many fundamental cellular processes. Of the various secretion systems, the Sec system is composed of seven different subunits in bacteria, and subunit SecB brings secreted preproteins to subunit SecA, which with SecYEG and SecDF forms a complex for the translocation of secreted preproteins through the inner membrane. Because of the wide existence of Sec system across bacteria, eukaryota, and archaea, each subunit of the Sec system has a complicated evolutionary relationship. Until very recently, 5,162 SecB sequences have been documented in UniProtKB, however no phylogenetic study has been conducted on a large sampling of SecBs from bacterial Sec secretion system, and no statistical study has been conducted on such size of SecBs in order to exhaustively investigate their variances of pairwise p-distance along taxonomic lineage from kingdom to phylum, to class, to order, to family, to genus and to organism. To fill in these knowledge gaps, 3,813 bacterial SecB sequences with full taxonomic lineage from kingdom to organism covering 4 phyla, 11 classes, 41 orders, 82 families, 269 genera, and 3,744 organisms were studied. Phylogenetic analysis revealed how the SecBs evolved without compromising their function with examples of 3-D structure comparison of two SecBs from Proteobacteria, and possible factors that affected the SecB evolution were considered. The average pairwise p-distances showed that the variance varied greatly in each taxonomic group. Finally, the variance was further partitioned into inter- and intra-clan variances, which could correspond to vertical and horizontal gene transfers, with relevance for Achromobacter, Brevundimonas, Ochrobactrum, and Pseudoxanthomonas.
Highlights
All living cells have the capacity to exchange material and information with their surrounding environment
The protein secretion system has been the focus of many studies, and is named in various ways at different times as seen in various databases and literature such as export membrane protein, protein-export membrane protein, preprotein translocase, protein translocase, secretion system, secretory pathway, translocase nanomachine, translocon, transmembrane export protein, etc
The SecB P0AG86 came from Escherichia coli belonging to the phylum Proteobacteria, class
Summary
All living cells have the capacity to exchange material and information with their surrounding environment. The protein secretion system is very different from the transport systems that transport water, inorganic ions, and small organic molecules because a cell can purposely synthesize different proteins, whose sizes and conformations can vary one from another. Because of this importance, the protein secretion system has been the focus of many studies, and is named in various ways at different times as seen in various databases and literature such as export membrane protein, protein-export membrane protein, preprotein translocase, protein translocase, secretion system, secretory pathway, translocase nanomachine, translocon, transmembrane export protein, etc. For such an important cellular system, it is not yet known as to when it first appeared in cells, and thereafter how it has evolved up to the present time
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