Cryo-EM structure of a functional monomeric Photosystem I from Thermosynechococcus elongatus reveals red chlorophyll cluster

A high-resolution structure of trimeric cyanobacterial Photosystem I (PSI) from Thermosynechococcus elongatus was reported as the first atomic model of PSI almost 20 years ago. However, the monomeric PSI structure has not yet been reported despite long-standing interest in its structure and extensive spectroscopic characterization of the loss of red chlorophylls upon monomerization. Here, we describe the structure of monomeric PSI from Thermosynechococcus elongatus BP-1. Comparison with the trimer structure gave detailed insights into monomerization-induced changes in both the central trimerization domain and the peripheral regions of the complex. Monomerization-induced loss of red chlorophylls is assigned to a cluster of chlorophylls adjacent to PsaX. Based on our findings, we propose a role of PsaX in the stabilization of red chlorophylls and that lipids of the surrounding membrane present a major source of thermal energy for uphill excitation energy transfer from red chlorophylls to P700. Coruh, Frank et al. report the structure of monomeric Photosystem I from cyanobacteria Thermosynechoccocus elongatus BP-1. They assign monomerization-induced loss of red chlorophylls to a cluster of chlorophylls adjacent to PsaX. This study suggests a role of PsaX in the stabilization of red chlorophylls.

Automated summary

A study reports the structure of monomeric Photosystem I from cyanobacteria Thermosynechoccocus elongatus BP-1, revealing insights into monomerization-induced changes in both the central trimerization domain and the peripheral regions of the complex, as well as linking monomerization-induced loss of red chlorophylls to a cluster of chlorophylls adjacent to PsaX. The findings propose a role of PsaX in stabilizing red chlorophylls and suggest that lipids in the surrounding membrane play a major role in uphill excitation energy transfer from red chlorophylls to P700.