Abstract
The extended superfamily of chlorophyll a/b binding proteins comprises the Light-Harvesting Complex Proteins (LHCs), the Early Light-Induced Proteins (ELIPs) and the Photosystem II Subunit S (PSBS). The proteins of the ELIP family were proposed to function in photoprotection or assembly of thylakoid pigment-protein complexes and are further divided into subgroups with one to three transmembrane helices. Two small One-Helix Proteins (OHPs) are expressed constitutively in green plant tissues and their levels increase in response to light stress. In this study, we show that OHP1 and OHP2 are highly conserved in photosynthetic eukaryotes, but have probably evolved independently and have distinct functions in Arabidopsis. Mutations in OHP1 or OHP2 caused severe growth deficits, reduced pigmentation and disturbed thylakoid architecture. Surprisingly, the expression of OHP2 was severely reduced in ohp1 T-DNA insertion mutants and vice versa. In both ohp1 and ohp2 mutants, the levels of numerous photosystem components were strongly reduced and photosynthetic electron transport was almost undetectable. Accordingly, ohp1 and ohp2 mutants were dependent on external organic carbon sources for growth and did not produce seeds. Interestingly, the induction of ELIP1 expression and Cu/Zn superoxide dismutase activity in low light conditions indicated that ohp1 mutants constantly suffer from photo-oxidative stress. Based on these data, we propose that OHP1 and OHP2 play an essential role in the assembly or stabilization of photosynthetic pigment-protein complexes, especially photosystem reaction centers, in the thylakoid membrane.
Highlights
Photosynthetic energy conversion in plants and algae forms the basis for almost all life on earth
Two Distinct Types of OneHelix Proteins (OHPs) Are Present in Phylogenetic analyses based on the signature element of Early Light-Induced Proteins (ELIPs) sequences, the conserved transmembrane helices with a predicted chlorophyll (Chl) binding motive, had very little power to resolve the evolutionary origin of the sub-families of OHP and Stress Enhanced Proteins (SEPs)/Light Harvesting-Like proteins (LILs) proteins (Engelken et al, 2012)
We used a manually refined alignment of OHP and SEP/LIL sequences around the conserved transmembrane helix corresponding to the entire length of mature OHP1 to calculate a maximum likelihood tree (Figure 1 and Supplemental Figure 1)
Summary
Photosynthetic energy conversion in plants and algae forms the basis for almost all life on earth. More than 1.5 billion years ago, engulfment of a cyanobacterium by a eukaryotic host cell gave rise to chloroplasts and thereby photo-autotrophic eukaryotes (Archibald, 2015). During the co-evolution of plastids and their host cells, the major part of the organelle genome was transferred to the host nucleus, generating the need for protein import to maintain organelle structure and function (Zimorski et al, 2014). The photosynthetic eukaryotes split into several lineages, giving rise to the living Glaucophytes, Chloroplastida, One-Helix Proteins in Arabidopsis and Rhodophytes. In both the “green” and the “red” lineage, further endocytobiosis events gave rise to secondary or even tertiary plastids (Zimorski et al, 2014)
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