Structural insights into an inward proton-pumping rhodopsin.

Area of Science:

• Structural biology
• Biochemistry
• Microbial rhodopsins

Background:

• Microbial rhodopsins facilitate light-driven proton transport.
• Inward proton pumps, like xenorhodopsins (XeRs) and schizorhodopsins (SzRs), are less structurally characterized than outward pumps.
• Limited structural data hinders understanding of inward proton pump variations.

Purpose of the Study:

• To determine the crystal structure of Antarctic rhodopsin (AntR), a representative schizorhodopsin (SzR).
• To elucidate the structural basis of inward proton transport in SzRs.
• To compare AntR structure with other microbial rhodopsins.

Main Methods:

• X-ray crystallography of AntR under acidic and basic conditions.
• Structural analysis comparing AntR to known rhodopsin structures.
• Functional analysis involving truncation of the C-terminal helix.

Main Results:

• The AntR structure reveals short transmembrane helices and large cavities, similar to SzR4 but distinct from NsXeR and BcXeR.
• A unique long C-terminal α-helix in AntR covers the cytoplasmic proton pathway.
• Truncation of this helix reduced inward proton-pumping efficiency.
• AntR exhibits pH-dependent retinal configuration.

Conclusions:

• The AntR structure provides crucial insights into the conserved and unique features of SzRs.
• A putative inward proton transport mechanism for AntR is proposed based on structural and functional data.
• Structural variations among inward proton pumps, including AntR, are highlighted.