Three-Dimensional Architecture of the Granum-Stroma Thylakoid Membrane System Revealed by Electron Tomography

Abstract

We Investigated The Three-Dimensional Architecture Of Isolated Granal Thylakoid Membranes By Using High Voltage (1,200 Kv) Electron Tomography On 250 Nm Thick Parts Of Isolated Intact Thylakoid Preparations Fixed And Stained With Conventional Electron Microscopy Techniques. High Resolution Reconstructed Tomographic Images Were Obtained Which Clearly Resolved The Membranes And The Lumenal Spaces, And Thus Also The Connections Between The Stacked And Non-Stacked Thylakoid Membranes. Based On Our Data, And Also Using The Electron Tomography Data Of Chloroplasts Within Cryo-Immobilized, Freeze Substituted Leaves (Shimoni Et Al. 2005), We Propose A Refined Model Of The Granum-Stroma Assembly. The Model Takes Into Account The Following Structural Factors And Membrane Properties: (I) Size Differences Between The Stroma Exposed Sides Of The Two Photosystems And Also The Protrusion Of The Atp Synthase In The Stroma; (Ii) The Lateral Segregation Of The Complexes Governed By The Self-Assembly Of Photosystem Ii (Psii) And Its Main Light Harvesting Complexes, The Lhcii, And The Formation Of Large Domains Enriched In Lhcii And Psii; (Iii) The Capability Of These Domains Of Stacking, A Step Initiating The Granum Formation And Stabilizing The Membrane Organization With The Lateral Heterogeneity; (Iv) The Fusion And Overlap Of Membranes During Their Growth. These Factors Can Bring About The Self-Assembly Of A Quasi-Helical Organization Of The Granumstroma Contiguous Thylakoid Membrane System That Encloses A Single Interior Aqueous Phase.