Structural analysis of PSI-ACPI and PSII-ACPII supercomplexes from a cryptophyte alga Rhodomonas sp. NIES-2332

Area of Science:

• Photosynthesis research
• Structural biology
• Algal biochemistry

Background:

• Photosynthesis converts light energy to chemical energy using photosystems (PSI and PSII) complexed with light-harvesting proteins.
• Cryptophyte algae, like Rhodomonas, possess unique chlorophyll a/c proteins (ACPs) and phycobiliproteins in their light-harvesting complexes.

Purpose of the Study:

• To determine the high-resolution structures of PSI-ACPI and PSII-ACPII supercomplexes from Rhodomonas sp. NIES-2332.
• To compare these structures with previously reported cryptophyte photosystems and identify novel features.
• To elucidate the roles of lipids and water molecules in supercomplex assembly and function.

Main Methods:

• Purification of PSI-ACPI and PSII-ACPII supercomplexes.
• High-resolution cryo-electron microscopy (cryo-EM) analysis (2.08 Å for PSI, 2.17 Å for PSII).

Main Results:

• Detailed structures of PSI-ACPI and PSII-ACPII supercomplexes were resolved.
• Identified differences in pigment locations and subunit structures compared to other cryptophytes, suggesting altered energy transfer rates.
• Discovered novel lipids at antenna-core interfaces, potentially crucial for supercomplex stability.
• Observed conserved and unique water molecules around PSI iron-sulfur clusters.
• PSII structure lacked oxygen-evolving complex and Mn4CaO5 cluster (S-phase), while PSI structure showed PsaQ binding (L-phase), indicating co-existence of phases.

Conclusions:

• The high-resolution structures provide a detailed basis for understanding energy transfer and quenching in cryptophyte algae.
• The findings suggest that different growth phases (S-phase and L-phase) can coexist within cryptophyte cells.
• Novel lipids and water molecule arrangements play significant roles in the assembly and stabilization of photosystem supercomplexes.