Photosynthetic Proteins in Cyanobacteria: from Translocation to Assembly of Photosynthetic Complexes

Cyanobacteria are ancient prokaryotes known for their ability to perform oxygenic photosynthesis. The photoautotrophic lifestyle of these organisms requires the efficient synthesis, translocation and assembly of photosynthetic proteins into supramolecular photosynthetic complexes which are typically localised in the thylakoid membranes. Co- and posttranslational translocation of proteins across and into cyanobacterial membranes is carried out by multiple protein translocation systems. These systems, which include the general secretory pathway, the twin-arginine translocation pathway and the signal recognition pathway, are highly conserved across prokaryotes and organelles of endosymbiotic origin. However, cyanobacteria are unique in the requirement to target proteins to and across multiple membranes (cytoplasmic and thylakoid) with a single set of machinery for each translocation pathway. Therefore, one of the current challenges is to understand how photosynthetic proteins and cofactors are synthesized, targeted, and assembled into supramolecular photosynthetic complexes. Recent studies have provided an unprecedented view into thylakoid biogenesis and organisation of photosynthetic complexes in cyanobacteria; however, open questions regarding targeting mechanisms and the existence of a physical connection between membrane systems remain. This chapter will summarize the existing knowledge on protein trafficking in cyanobacteria with a focus on photosynthetic proteins. This information will then be contextualised to discuss similarities and differences between protein targeting in cyanobacteria, other prokaryotes and chloroplast thylakoid membranes.