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

Abstract

Magnesium cation (Mg2+) 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, Mg2+ is the central ion in chlorophylls, and Mg2+ 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 Mg2+ homoeostasis, transport and distribution within cyanobacteria. However, Mg2+ 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 Mg2+ channels CorA and/or MgtE, which both facilitate transmembrane Mg2+ flux down the electrochemical gradient. Both Mg2+ channels are typical for non-photosynthetic bacteria. Furthermore, Mg2+ transporters of the MgtA/B family are also present in the cytoplasmic membrane to mediate active Mg2+ import into the bacterial cell. While the cytoplasmic membrane of cyanobacteria resembles a ‘classical’ bacterial membrane, essentially nothing is known about Mg2+ channels and/or transporters in thylakoid membranes of cyanobacteria or chloroplasts. As discussed here, at least one Mg2+ channelling protein must be localized within thylakoid membranes. Thus, either one of the ‘typical’ bacterial Mg2+ channels has a dual localization in the cytoplasmic plus the thylakoid membrane, or another, yet unidentified channel is present in cyanobacterial thylakoid membranes.

Comparative Genomics of the Transport Proteins of Ten Lactobacillus Strains

“…Eight of the ten strains (excluding Lru and Lga) have a homologue of the CorA Metal Ion Transporter (MIT) Family (TC# 1.A.35). Members of this family are involved in the uptake of divalent cations, primarily Mg 2+ , but also Cd 2+ , Co 2+ , Ni 2+ , and Zn 2+ [ 33 , 34 ]. Interestingly, all ten strains (OP and PAP) have two homologs of each of camphor resistance proteins (TC# 1.A.43.1.1 and TC# 1.A.43.1.2).…”

Current knowledge and recent advances in understanding metabolism of the model cyanobacterium Synechocystis sp. PCC 6803

“…Atx1 may also act as a Zn 2+ chaperone, in addition to its role as a Cu 2+ chaperone [238]. The Synechocystis genome also encode two putative magnesium transport proteins, MgtC and MgtE [239], both of which localize to the PM [32].…”

Proton Leakage Is Sensed by IM30 and Activates IM30-Triggered Membrane Fusion

“…Direct binding of Mg 2+ to IM30 induces structural changes, such as increased exposure of hydrophobic surface regions, an overall more compact structure and increased protein stability [37]. The Mg 2+ concentration in the cyanobacterial cytoplasm and the chloroplast stroma is light-dependent and is related to the photosynthetic activity of chloroplasts and cyanobacteria [38], which directly links IM30-mediated membrane fusion processes to photosynthetic electron-transfer reactions.…”