Phycobilisome core architecture influences photoprotective quenching by the Orange Carotenoid Protein

Abstract

Photosynthetic organisms rely on sophisticated photoprotective mechanisms to prevent oxidative damage under high or fluctuating solar illumination. Cyanobacteria, which have evolved a modular, water-soluble light harvesting complex - the phycobilisome - achieve photoprotection through a unique, photoactivatable quencher called the Orange Carotenoid Protein (OCP). Although phycobiliproteins are highly conserved, phycobilisomes take on different macromolecular architectures in different species of cyanobacteria, and it is not well understood whether or how these structures relate to changes in photoprotective function. To learn whether OCP functions similarly across species with different core architectures, we experimentally compare the photophysical states accessible to prototypical tricylindrical and pentacylindrical phycobilisomes, with and without OCP, at the single-molecule level using an Anti-Brownian ELectrokinetic (ABEL) trap. We compare our data to Monte Carlo simulations of exciton transfer in compartmental models of phycobilisomes with OCP bound at different combinations of predicted docking sites. Our results suggest that while some aspects of OCP function are influenced by phycobilisome architecture, others are surprisingly well-conserved: OCP appears to bind at different locations in each architecture and cross-species OCP-phycobilisome compatibility is asymmetric, yet the quenching strength and dimeric binding of OCP appear to be similar for both phycobilisome architectures. Together, our findings provide new insights into how the uniquely modular architecture of phycobilisomes enables robust conservation as well as fine-tuning of the OCP quenching mechanism across species.