Genomic insights into cyanobacterial protein translocation systems

Abstract

Cyanobacteria are ubiquitous oxygenic photosynthetic bacteria with a versatile metabolism that is highly dependent on effective protein targeting. Protein sorting in diderm bacteria is not trivial and, in cyanobacteria, even less so due to the presence of a complex membrane system: the outer membrane, the plasma membrane and the thylakoid membrane. In cyanobacteria, protein import into the thylakoids is essential for photosynthesis, export to the periplasm fulfills a multifunctional role in maintaining cell homeostasis, and secretion mediates motility, DNA uptake and environmental interactions. Intriguingly, only one set of genes for the general secretory and the twin-arginine translocation pathways seem to be present. However, these systems have to operate in both plasma and thylakoid membranes. This raises the question of how substrates are recognized and targeted to their correct, final destination. Additional complexities arise when a protein has to be secreted across the outer membrane, where very little is known regarding the mechanisms involved. Given their ecological importance and biotechnological interest, a better understanding of protein targeting in cyanobacteria is of great value. This review will provide insights into the known knowns of protein targeting, propose hypotheses based on available genomic sequences and discuss future directions.

Genome-Wide Identification and Bioinformatics Characterization of Superoxide Dismutases in the Desiccation-Tolerant Cyanobacterium Chroococcidiopsis sp. CCMEE 029

“…The presence of only one TatC was in agreement with the results of a survey of 85 cyanobacterial genomes revealing that half of them, including many heterocystforming cyanobacteria, have two TatA/B homologs, and the other half, including most of the marine picocyanobacteria, have one TatA/B homolog (Russo and Zedler, 2020). As reported for other cyanobacteria (Russo and Zedler, 2020), the two TatA from Chroococcidiopsis sp. CCMEE 029 were highly homologous to each other, and therefore, it was impossible to establish whether it encodes a TatABC or a minimal TatAC system (Russo and Zedler, 2020).…”

“…CCMEE 029's genome identified three genes encoding Tat machinery proteins: two TatA and one TatC. The presence of only one TatC was in agreement with the results of a survey of 85 cyanobacterial genomes revealing that half of them, including many heterocystforming cyanobacteria, have two TatA/B homologs, and the other half, including most of the marine picocyanobacteria, have one TatA/B homolog (Russo and Zedler, 2020). As reported for other cyanobacteria (Russo and Zedler, 2020), the two TatA from Chroococcidiopsis sp.…”

“…As reported for other cyanobacteria (Russo and Zedler, 2020), the two TatA from Chroococcidiopsis sp. CCMEE 029 were highly homologous to each other, and therefore, it was impossible to establish whether it encodes a TatABC or a minimal TatAC system (Russo and Zedler, 2020). Furthermore, the two ChTatA proteins exhibited residues that are exclusive to either TatA or TatB in E. coli (Hicks et al, 2003.…”

“…In both the Sec and Tat machinery proteins are targeted for export by a signal sequence characterized by a basic n-region, a longer h-region of hydrophobic amino acids and a c-region with positively charged amino acids, the so-called Sec-avoidance motif, and the peptidase-recognition sequence for signal sequence removal and release of the mature protein (Frain et al, 2016). The genome mining of 85 cyanobacteria revealed that the majority encode a single gene set of the Sec system, whereas homologs of the Tat systems were identified in all the surveyed genomes (Russo and Zedler, 2020). After the transport via the Sec or Tat system, protein sorting to a different localization occurs through the signal recognition particle (SRP) pathway or via spontaneous insertion (Frain et al, 2016).…”

The Role of the Cyanobacterial Type IV Pilus Machinery in Finding and Maintaining a Favourable Environment

“…Involvement of the T4P apparatus in secretion is not without precedent in cyanobacteria. Secretion of heterologous proteins in Synechococcus elongatus and other cyanobacteria has been suggested to proceed via the T4P apparatus by an unknown mechanism [105,106]. Furthermore, accumulation of extracellular PilA in N. punctiforme is connected with polysaccharide secretion via HmpF, PilB, and PilQ [6].…”