Fast-activating channel controls cation fluxes across the native chloroplast envelope

Area of Science:

• Plant Physiology
• Molecular Biology
• Biophysics

Background:

• Photosynthetic carbon dioxide fixation requires alkaline pH in the chloroplast stroma.
• Stromal alkalinization is achieved by proton pumping, necessitating cation influx to maintain charge balance.
• The specific channels responsible for cation influx in chloroplast envelopes remained unidentified.

Purpose of the Study:

• To identify and characterize cation channels in the chloroplast envelope of Pisum sativum (pea).
• To investigate the functional properties and regulation of these channels.
• To determine the role of these channels in supporting photosynthetic CO(2) fixation.

Main Methods:

• Utilized the patch-clamp technique on isolated pea chloroplasts.
• Investigated channel activity under varying electrical potentials and stromal pH.
• Examined the effect of gadolinium ions (Gd(3+)) on channel function.

Main Results:

• Discovered a fast-activating chloroplast cation (FACC) channel in the native chloroplast envelope.
• FACC channels exhibit voltage-dependent gating, opening rapidly at large positive or negative potentials.
• Channel conductance significantly increases with applied voltage, and it conducts essential inorganic cations (K+, Na+, Ca2+, Mg2+).
• Increased stromal pH (mimicking light conditions) reduces FACC channel activity.
• FACC channels are irreversibly blocked by Gd(3+).

Conclusions:

• The identified FACC channels are essential for cation influx required to balance proton pumping during photosynthesis.
• These channels likely mediate electroneutral K+/H+ exchange, facilitating stromal alkalinization.
• The regulation of FACC channels by stromal pH and voltage is critical for optimizing photosynthetic performance.