The Role of Outer Membrane Protein(s) Harboring SLH/OprB-Domains in Extracellular Vesicles’ Production in Synechocystis sp. PCC 6803

Abstract

Cyanobacteria are a group of photosynthetic prokaryotes that contribute to primary production on a global scale. These microorganisms release vesicles to the extracellular environment, spherical nanosized structures, derived essentially from the outer membrane. Even though earlier works in model Gram-negative bacteria have hypothesized that outer membrane stability is crucial in vesicle formation, the mechanisms determining vesicle biogenesis in cyanobacteria remain unknown. Here, we report on the identification of six candidate genes encoding outer membrane proteins harboring SLH/OprB-domains in the genome of the model cyanobacterium Synechocystis sp. PCC 6803. Using a genetics-based approach, one gene was found to encode an essential protein (Slr1841), while the remaining five are not essential for growth under standard conditions. Vesicle production was monitored, and it was found that a mutant in the gene encoding the second most abundant SLH/OprB protein in Synechocystis sp. PCC 6803 outer membrane (Slr1908) produces more vesicles than any of the other tested strains. Moreover, the Slr1908-protein was also found to be important for iron uptake. Altogether, our results suggest that proteins containing the SLH/OprB-domains may have dual biological role, related to micronutrient uptake and to outer membrane stability, which, together or alone, seem to be involved in cyanobacterial vesicle biogenesis.

Extracellular Vesicle-Mediated Secretion of Protochlorophyllide in the Cyanobacterium Leptolyngbya boryana

“…TolC, an efflux protein common to RND-type transporters and the type I secretion system (T1SS) [ 25 , 26 ], is localized to the outer membrane. Cyanobacterial porin, the SLH/OprB-domain-containing protein, is also localized to the outer membrane [ 20 ]. Thus, Fractions 1 and 2 contained mainly outer membrane-localized proteins, indicating that Pchlide is secreted via EVs probably derived from the outer membrane of YFC2.…”

“…These fractions also differed in protein composition, with Fraction 2 having porin isoforms as major proteins, a feature similar to that of the outer membrane fraction of another cyanobacterium Synechocystis sp. PCC 6803 (two porin isoforms as major proteins) [ 20 ]. However, no genes encoding FG-GAP repeat-containing protein-homologs were discovered in the genome of Synechocystis sp.…”

“…The genome of Synechocystis sp. PCC 6803 contains six genes encoding porin isoforms, and the RNA-seq database indicated that three of these genes, which were detected in outer membrane fractions in this organism, were primarily expressed [ 20 ]. In the genome of L. boryana , 16 genes were discovered when BLAST search was performed using the amino acid sequence of Slr1908, the major porin isoform of Synechocystis sp.…”

“…EV formation has been discovered in several cyanobacteria, including the model cyanobacterium Synechocystis sp. PCC 6803 [ 19 , 20 ]. The roles of cyanobacterial EVs have been suggested to include adaptive responses to various environmental stresses, defense against cyanophages, horizontal gene transfer, and biofilm formation [ 20 ].…”

“…PCC 6803 [ 19 , 20 ]. The roles of cyanobacterial EVs have been suggested to include adaptive responses to various environmental stresses, defense against cyanophages, horizontal gene transfer, and biofilm formation [ 20 ]. However, the involvement of EVs in Chl biosynthesis is yet to be reported.…”