TLP18.3, a novel thylakoid lumen protein regulating photosystem II repair cycle

Abstract

A proteome analysis of Arabidopsis thaliana thylakoid-associated polysome nascent chain complexes was performed to find novel proteins involved in the biogenesis, maintenance and turnover of thylakoid protein complexes, in particular the PSII (photosystem II) complex, which exhibits a high turnover rate. Four unknown proteins were identified, of which TLP18.3 (thylakoid lumen protein of 18.3 kDa) was selected for further analysis. The Arabidopsis mutants (SALK_109618 and GABI-Kat 459D12) lacking the TLP18.3 protein showed higher susceptibility of PSII to photoinhibition. The increased susceptibility of DeltaTLP18.3 plants to high light probably originates from an inefficient reassembly of PSII monomers into dimers in the grana stacks, as well as from an impaired turnover of the D1 protein in stroma exposed thylakoids. Such dual function of the TLP18.3 protein is in accordance with its even distribution between the grana and stroma thylakoids. Notably, the lack of the TLP18.3 protein does not lead to a severe collapse of the PSII complexes, suggesting a redundancy of proteins assisting these particular repair steps to assure functional PSII. The DeltaTLP18.3 plants showed no clear visual phenotype under standard growth conditions, but when challenged by fluctuating light during growth, the retarded growth of DeltaTLP18.3 plants was evident.

Structural and Functional Assays of AtTLP18.3 Identify Its Novel Acid Phosphatase Activity in Thylakoid Lumen

“…AtTLP18.3 is one of the proteins associated with the polysome, and only a low amount can be found in the thylakoid preparation. Furthermore, the amount of AtTLP18.3 protein remains constant and is not regulated by light (Sirpiö et al, 2007).…”

“…In the past several years, different proteomic approaches have revealed hundreds of chloroplast proteins, including those with unknown function, located at the thylakoid membrane and lumen (Schubert et al, 2002;Friso et al, 2004;Goulas et al, 2006;Sirpiö et al, 2007). These proteomic studies provide a detailed overview of photosynthetic apparatuses and the auxiliary proteins for maintaining the photosystem in the chloroplast.…”

“…2A). Because the phenotype of AtTLP18.3 could not be observed by knockout or overexpression experiments under normal growth conditions (Sirpiö et al, 2007), investigating the molecular structure suggested an alternative approach to explore its function. Therefore, we aimed to resolve the structure of AtTLP18.3 by x-ray crystallography and propose its molecular function by comparing the structure with the known functions of other proteins (Lee et al, 2001).…”

“…In previous studies, the knockout mutant of AtTLP18.3 did not show an observable phenotype under normal growth conditions but showed retarded growth under fluctuating light (Sirpiö et al, 2007;Mulo et al, 2008). The protein was proposed to provide partial protection for the PSII complex to resist photoinhibition under fluctuating high-light conditions (Sirpiö et al, 2007). Furthermore, except for research involving microarrays and proteomics that has alluded to this gene, no research has focused on the function of this gene.…”

“…The mature form of AtTLP18.3 consists of DUF477 and TMH for anchoring at the thylakoid membrane. In previous studies, the knockout mutant of AtTLP18.3 did not show an observable phenotype under normal growth conditions but showed retarded growth under fluctuating light (Sirpiö et al, 2007;Mulo et al, 2008). The protein was proposed to provide partial protection for the PSII complex to resist photoinhibition under fluctuating high-light conditions (Sirpiö et al, 2007).…”