Loss of ALBINO3b Insertase Results in Truncated Light-Harvesting Antenna in Diatoms

Abstract

The family of chloroplast ALBINO3 (ALB3) proteins function in the insertion and assembly of thylakoid membrane protein complexes. Loss of ALB3b in the marine diatom Phaeodactylum tricornutum leads to a striking change of cell color from the normal brown to green. A 75% decrease of the main fucoxanthin-chlorophyll a/c-binding proteins was identified in the alb3b strains as the cause of changes in the spectral properties of the mutant cells. The alb3b lines exhibit a truncated light-harvesting antenna phenotype with reduced amounts of light-harvesting pigments and require a higher light intensity for saturation of photosynthesis. Accumulation of photoprotective pigments and light-harvesting complex stress-related proteins was not negatively affected in the mutant strains, but still the capacity for nonphotochemical quenching was lower compared with the wild type. In plants and green algae, ALB3 proteins interact with members of the chloroplast signal recognition particle pathway through a Lys-rich C-terminal domain. A novel conserved C-terminal domain was identified in diatoms and other stramenopiles, questioning if ALB3b proteins have the same interaction partners as their plant/green algae homologs.

Toward Enhanced Fixation of CO2 in Aquatic Biomass: Focus on Microalgae

“…Conversely, when limiting light was supplied, TLA growth was reduced compared to the WT one (Nakajima et al, 2001;Perrine et al, 2012;Shin et al, 2017). In addition, some TLA mutants showed null advantages in their productivity, irrespective of the condition tested (Page et al, 2012;Nymark et al, 2019). Diverging phenotypes among TLA mutants have been ascribed to undesirable side effects of the mutation or to an unbalanced photoprotection caused by the altered antenna system (de Mooij et al, 2015;Nymark et al, 2019).…”

“…In addition, some TLA mutants showed null advantages in their productivity, irrespective of the condition tested (Page et al, 2012;Nymark et al, 2019). Diverging phenotypes among TLA mutants have been ascribed to undesirable side effects of the mutation or to an unbalanced photoprotection caused by the altered antenna system (de Mooij et al, 2015;Nymark et al, 2019). In TLA Synechocystis cells, the reduced size of phycobilisomes was also shown to have a wide impact on cell proteome (Liberton et al, 2017).…”

Functional studies of CpSRP54 in diatoms show that the mechanism of thylakoid protein insertion differs from that in plants and green algae

“…Arabidopsis thaliana CpSRP54 co‐translationally associates with a range of different subunits of the PET chain complexes, but a similar function has not been reported in C. reinhardtii (Nilsson et al ., 1999; Nilsson and van Wijk, 2002; Piskozub et al ., 2015; Jeong et al ., 2017; Hristou et al ., 2019). In diatoms, homologs of CpSRP54, CpFTSY, and two variants of ALB3 (ALB3a and ALB3b) have been identified, whereas CpSRP43 homologs are absent (Träger et al ., 2012; Nymark et al ., 2019). In addition, the LHCP translocation defect protein (LTD), which forwards LHCPs from the chloroplast import machinery to the CpSRP complex in plants and green algae, is apparently also missing in diatom genomes (Ouyang et al ., 2011; Jeong et al ., 2018).…”

“…The function of ALB3b was recently investigated in the model diatom Phaeodactylum tricornutum, where loss of the insertase caused a striking change in cell color from brown to green (Nymark et al ., 2019). The change in pigmentation was caused by an approximately 75% decrease of the main group of FCP proteins (LHCFs) and a subsequent lowered level of antenna pigments (including Fx, which provides the brown color of diatoms).…”

“…However, the ALB3b protein does not contain the basic lysine‐rich C‐terminal domain (CTD) from plants known to interact with other members of the CpSRP pathway (Falk et al ., 2010; Horn et al ., 2015; Chandrasekar and Shan, 2017). Instead, a CTD conserved within the group of diatoms was identified in the ALB3b insertase, but its protein interaction partners are unknown (Nymark et al ., 2019). ALB3a does contain the basic lysine‐rich CTD, but repeated attempts to create KO mutants of this insertase were unsuccessful, implying a crucial function in the co‐translational CpSRP pathway similar to ALB3 and ALB3.2 of plants and green algae, respectively (Sundberg et al ., 1997; Göhre et al ., 2006; Nymark et al ., 2019).…”