Putting it together: CEPA1 functions as a Photosystem I assembly factor

Background:

Photosystem I (PSI) is a large pigment-protein complex that participates in photosynthetic electron transfer. While its structure is well resolved, its assembly pathway is less clear.  A set of proteins mediating the step-wise assembly of PSI subunits, known as PSI assembly factors, has been previously characterized, but many key players are likely still missing.  as an entry point into the discovery of yet unknown PSI assembly factors, we screened for genes that are co-expressed with known PSI assembly factor-encoding genes in Arabidopsis.  This approach led to the identification of CO-EXPRESSED WITH PSI ASSEMBLY1 (CEPA1).

Questions:

What is the fate of PSI when CEPA1 is absent? How does CEPA1 function?

Findings:

CEPA1 is targeted to the thylakoid membranes where PSI accumulates.  Arabidopsis mutants lacking CEPA1 grow autotrophically but suffer from delayed development and exhibit pigment deficiency. This phenotype is caused by a strong decrease in PSI content.  CEPA1 does not regulate plastid PSI gene expression, but instead, acts at the post-translational level. CEPA1 associates with PSI assembly intermediates in the thylakoid membrane, and interacts with the PSI assembly factor PHOTOSYSTEM 1 ASSEMBLY3 (PSA3). The currently available data suggest a model in which CEPA1 is cleaved out of the nascent PSI complexes once its assembly function is fulfilled.

Next steps:

The precise role of CEPA1 in PSI assembly will be further studies, in particular by investigating the relationship between CEPA1 and PSA3, and by characterizing CEPA1-containing protein complexes.

Reference:

David Rolo, Omar Sandoval-Ibáñez, Wolfram Thiele, Mark A Schöttler, Ines Gerlach, Reimo Zoschke, Joram Schwartzmann, Etienne H Meyer, Ralph Bock (2024) CO-EXPRESSED WITH PSI ASSEMBLY1 (CEPA1) is a photosystem I assembly factor in Arabidopsis https://doi.org/10.1093/plcell/koae042