Determining the Macromolecular Structures of Photosynthetic Supercomplexes from Rhodobacter using Cryo-Electron Microscopy

Bracun, Laura ORCID: 0000-0001-7698-3718 (2023) Determining the Macromolecular Structures of Photosynthetic Supercomplexes from Rhodobacter using Cryo-Electron Microscopy. PhD thesis, University of Liverpool.

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Abstract

The reaction center (RC)−light-harvesting complex 1 (LH1) supercomplex plays a central role in bacterial photosynthesis by converting light into chemical energy. Some RC–LH1 complexes integrate an additional protein PufX which is key for bacterial growth and photosynthetic competence. This thesis presents the atomic models of RC−LH1−PufX complexes from three species of purple bacteria: Rhodobacter (Rba.) sphaeroides, Rba. veldkampii and Rba. capsulatus. Electron potential maps used to build the models were obtained by single particle cryo-electron microscopy at resolutions of 2.84 Å (Rba. veldkampii), 3.40 Å and 2.79 Å (Rba. sphaeroides), and 2.59 Å (Rba. capsulatus). The RC–LH1–PufX native monomers of Rba. veldkampii and Rba. capsulatus contain LH1 rings comprised of 15 αβ-heterodimers featuring a gap formed by PufX. The RC−LH1−PufX native dimer of Rba. sphaeroides contains an S-shaped LH1 ring of 28 αβ-heterodimers with two large gaps formed by PufX polypeptides, and an additional polypeptide PufY inserted between the LH1 ring and the RC near each gap. In order to understand the process of dimerization and the structural impacts of the additional polypeptides PufX and PufY, we also characterized the native monomer and three mutant complexes: one without the PufX, one without the PufY and one with PufX replaced with PufX of a strictly monomeric complex from Rba. veldkampii. The presented high-resolution WT and mutant structures provided insights into monomeric and dimeric RC‒LH1 complex formation, quinone pathway and roles of polypeptides PufX and PufY. PufX was observed to mediate dimerization through its N-terminus, correct complex assembly and halting the LH1 ring elongation resulting in a characteristic open architecture. PufY was observed to possibly stabilize the LH1 ring, increasing the efficiency of excitation energy transfer and potentially helping with quinone navigation. The features, similarities and differences displayed by RC‒LH1‒PufX complexes serve as a structural basis for understanding the mechanism of anoxygenic photosynthesis, as well as environmental adaptation and evolution of photosynthetic systems.

Item Type:	Thesis (PhD)
Divisions:	Faculty of Health and Life Sciences
Depositing User:	Symplectic Admin
Date Deposited:	31 Jan 2024 11:55
Last Modified:	31 Jan 2024 11:55
DOI:	10.17638/03171771
Supervisors:	Liu, Luning
URI:	https://livrepository.liverpool.ac.uk/id/eprint/3171771