Mg2+ homeostasis and transport in cyanobacteria – at the crossroads of bacterial and chloroplast Mg2+ import

Abstract

Magnesium cation (Mg 2+) is the most abundant divalent cation in living cells, where it is required for various intracellular functions. In chloroplasts and cyanobacteria, established photosynthetic model systems, Mg 2+ is the central ion in chlorophylls, and Mg 2+ flux across the thylakoid membrane is required for counterbalancing the light-induced generation of a ΔpH across the thylakoid membrane. Yet, not much is known about Mg 2+ homoeostasis, transport and distribution within cyanobacteria. However, Mg 2+ transport across membranes has been studied in non-photosynthetic bacteria, and first observations and findings are reported for chloroplasts. Cyanobacterial cytoplasmic membranes appear to contain the well-characterized Mg 2+ channels CorA and/or MgtE, which both facilitate transmembrane Mg 2+ flux down the electrochemical gradient. Both Mg 2+ channels are typical for non-photosynthetic bacteria. Furthermore, Mg 2+ transporters of the MgtA/B family are also present in the cytoplasmic membrane to mediate active Mg 2+ import into the bacterial cell. While the cytoplasmic membrane of cyanobacteria resembles a ‘classical’ bacterial membrane, essentially nothing is known about Mg 2+ channels and/or transporters in thylakoid membranes of cyanobacteria or chloroplasts. As discussed here, at least one Mg 2+ channelling protein must be localized within thylakoid membranes. Thus, either one of the ‘typical’ bacterial Mg 2+ channels has a dual localization in the cytoplasmic plus the thylakoid membrane, or another, yet unidentified channel is present in cyanobacterial thylakoid membranes.