The photosystem I assembly apparatus consisting of Ycf3\u2013Y3IP1 and Ycf4 modules

Sreedhar Nellaepalli,Hiroshi Kuroda,Shin-Ichiro Ozawa,Yuichiro Takahashi

doi:10.1038/s41467-018-04823-3

Abstract

In oxygenic photosynthesis, light energy is converted into redox energy by two photosystems (PSI and PSII). PSI forms one of the largest multiprotein complexes in thylakoid membranes consisting of a core complex, peripheral light-harvesting complexes (LHCIs) and cofactors. Although the high-resolution structure of the PSI–LHCI complex has been determined, the assembly process remains unclear due to the rapid nature of the assembly process. Here we show that two conserved chloroplast-encoded auxiliary factors, Ycf3 and Ycf4, form modules that mediate PSI assembly. The first module consists of the tetratricopeptide repeat protein Ycf3 and its interacting partner, Y3IP1, and mainly facilitates the assembly of reaction center subunits. The second module consists of oligomeric Ycf4 and facilitates the integration of peripheral PSI subunits and LHCIs into the PSI reaction center subcomplex. We reveal that these two modules are major mediators of the PSI–LHCI assembly process.

Highlights

In oxygenic photosynthesis, light energy is converted into redox energy by two photosystems (PSI and PSII)
It has been difficult to characterize the assembly mechanism because photosystem I (PSI) complex assembly proceeds rapidly and assembly intermediates accumulate in small amounts[8]
The present study revealed that the Ycf3–Ycf3-interacting protein 1 (Y3IP1) module transiently binds newly synthesized reaction center (RC) subunits, indicating that this module plays an essential role in an initial step of the PSI RC assembly

Summary

Introduction

Light energy is converted into redox energy by two photosystems (PSI and PSII). We show that two conserved chloroplast-encoded auxiliary factors, Ycf[3] and Ycf[4], form modules that mediate PSI assembly. The second module consists of oligomeric Ycf[4] and facilitates the integration of peripheral PSI subunits and LHCIs into the PSI reaction center subcomplex We reveal that these two modules are major mediators of the PSI–LHCI assembly process. Ycf[3] is an extrinsic protein associated with the thylakoid membranes and is essential for PSI complex biogenesis in C. reinhardtii[9] and tobacco[20]. This contains three tetratricopeptide repeat (TPR) domains, which could be involved in mediating the interaction between proteins. We carried out affinity purification of these HA-tagged proteins from the solubilized thylakoid membranes and characterized interacting proteins including PSI assembly intermediates

Methods

Results

Conclusion