Structure-Based Functional Modification Study of a Cyanobacterial Chloride Pump for Transporting Multiple Anions

Area of Science:

• Structural biology
• Biochemistry
• Microbial rhodopsins

Background:

• Cyanobacterial halorhodopsins are vital for ion transport.
• Synechocystis halorhodopsin (SyHR) transports both chloride and divalent ions like sulfate.
• Mastigocladopsis repens halorhodopsin (MrHR) is a homolog of SyHR.

Purpose of the Study:

• Determine the crystal structure of MrHR.
• Elucidate the structural basis for anion selectivity in cyanobacterial halorhodopsins.
• Understand the mechanism of divalent ion transport by SyHR.

Main Methods:

• X-ray crystallography at 1.9 Å resolution.
• Structural analysis of MrHR homotrimer.
• Alanine mutagenesis of key residues (Asn63, Pro118, Glu182).

Main Results:

• Detailed structure of MrHR revealed a unique trimeric topology and a "TSD" chloride ion conduction motif.
• Identified key residues (Asn63, Pro118, Glu182) responsible for anion selectivity, enabling transport of chloride, nitrate, and sulfate.
• Structural insights into how SyHR facilitates divalent ion transport were obtained.

Conclusions:

• The MrHR structure provides a framework for understanding cyanobacterial chloride pumps.
• Findings advance knowledge of microbial rhodopsins with diverse motifs.
• This research clarifies the molecular mechanisms of SyHR's unique ion transport capabilities.