Antenna Protein Clustering In Vitro Unveiled by Fluorescence Correlation Spectroscopy

Area of Science:

• Plant biology
• Biophysics
• Photosynthesis research

Background:

• Antenna protein aggregation is a key mechanism for photoprotection in plants against high light stress.
• In vitro studies often mimic this by altering detergent and pH, but the size and organization of resulting particles and their physiological relevance are unclear.

Purpose of the Study:

• To investigate the size, homogeneity, and fluorescence of antenna protein (LHCII) aggregates formed in vitro under varying pH conditions.
• To determine the physiological validity of in vitro quenching experiments for understanding in vivo non-photochemical quenching.

Main Methods:

• Utilized fluorescence correlation spectroscopy (FCS), a quasi-single molecule technique.
• Applied FCS to in vitro quenched LHCII trimers from higher plants to simultaneously measure particle size, fluorescence, and homogeneity.
• Varied pH conditions during the in vitro quenching process.

Main Results:

• Low pH (below critical micelle concentration) induced large, micrometer-sized protein oligomers, deemed incompatible with thylakoid membranes.
• High pH resulted in smaller, homogenous LHCII clusters that were still capable of efficient quenching.
• FCS provided parallel estimation of particle size, fluorescence, and homogeneity in a single measurement.

Conclusions:

• Established the physiological validity limits for in vitro quenching experiments by demonstrating incompatibility of low pH-induced aggregates with in vivo conditions.
• Supported the hypothesis that small, moderately quenching LHCII oligomers formed at high pH are physiologically relevant for in vivo non-photochemical quenching.