Genetic and Physical Interaction Studies Reveal Functional Similarities between ALBINO3 and ALBINO4 in Arabidopsis  

Abstract

ALBINO3 (ALB3) is a well-known component of a thylakoid protein-targeting complex that interacts with the chloroplast signal recognition particle (cpSRP) and the cpSRP receptor, chloroplast filamentous temperature-sensitive Y (cpFtsY). Its proteininserting function has been established mainly for light-harvesting complex proteins, which first interact with the unique chloroplast cpSRP43 component and then are delivered to the ALB3 integrase by a GTP-dependent cpSRP-cpFtsY interaction. In Arabidopsis (Arabidopsis thaliana), a subsequently discovered ALB3 homolog, ALB4, has been proposed to be involved not in light-harvesting complex protein targeting, but instead in the stabilization of the ATP synthase complex. Here, however, we show that ALB3 and ALB4 share significant functional overlap, and that both proteins are required for the efficient insertion of cytochrome f and potentially other subunits of pigment-bearing protein complexes. Genetic and physical interactions between ALB4 and ALB3, and physical interactions between ALB4 and cpSRP, suggest that the two ALB proteins may engage similar sets of interactors for their specific functions. We propose that ALB4 optimizes the insertion of thylakoid proteins by participating in the ALB3-cpSRP pathway for certain substrates (e.g. cytochrome f and the Rieske protein). Although ALB4 has clearly diverged from ALB3 in relation to the partner-recruiting C-terminal domain, our analysis suggests that one putative cpSRP-binding motif has not been entirely lost.

Suppressors of the Chloroplast Protein Import Mutant tic40 Reveal a Genetic Link between Protein Import and Thylakoid Biogenesis

“…In relation to stic1, these observations are consistent with those made by Gerdes et al (2006) and Trösch et al (2015) using the alb4-1 T-DNA insertion mutant. The appearance of the stic mutant chloroplasts is reminiscent of the previously described swollen chloroplast phenotype of tic40 plants (Kovacheva et al, 2005;Bédard et al, 2007), although in that case the thylakoid membrane system was much less developed.…”

“…This suggests that the loss of STIC proteins may be compensated for by redundancy or the operation of parallel pathways or that STIC functions become critical only under certain circumstances. The fact that the stic1-1 mutation was not found to have a visible effect on plant development is in agreement with the findings of Gerdes et al (2006) and Trösch et al (2015), who characterized the alb4-1 mutant, but in contrast to the stunted growth phenotype observed by Benz et al (2009).…”

“…It was recently shown that the alb4-1 mutation interacts synergistically with the alb3-1 null mutation, producing seedlings that are even more chlorotic and weak than the single alb3 mutant (Trösch et al, 2015). This was interpreted to indicate that there is a degree of functional redundancy between the two affected proteins and that ALB4 participates in the cpSRP pathway or in a parallel pathway that performs a similar role.…”

“…The ALB3 protein then mediates the insertion (and assembly) of the LHCP protein into the thylakoid membrane (Moore et al, 2000;Falk et al, 2010;Dünschede et al, 2011). Accordingly, Arabidopsis alb3 mutants display a strongly chlorotic/albino phenotype linked to defective LHCP biogenesis (Sundberg et al, 1997;Asakura et al, 2008); by contrast, those with defects affecting a second, paralogous protein called ALB4 appear relatively normal, suggesting that this factor is functionally different (Gerdes et al, 2006;Benz et al, 2009;Trösch et al, 2015). Recently, a novel component of the cpSRP pathway was identified: The LHCP targeting deficient (LTD) protein acts upstream of the cpSRP by mediating the transfer of the newly imported LHCP proteins from the TIC translocon to the cpSRP complex (Ouyang et al, 2011).…”

“…Further fine-mapping and sequencing, using the stic1-4 and stic2-1 alleles only, revealed potential splice-defect mutations in ALB4 (G to A at nucleotide 920 relative to the ATG) for stic1-4 ( Figure 2A) and in a gene with unknown function (At2g24020; G to A at nucleotide 670 relative to the ATG) for stic2-1 ( Figure 2B). ALB4 is a paralog of the ALB3 thylakoid membrane insertase and has been proposed to cooperate with ALB3 in the insertion, assembly, or stabilization of thylakoid membrane proteins (Benz et al, 2009;Trösch et al, 2015). Both ALB4 and ALB3 are members of the conserved Alb3/Oxa1/YidC family of protein insertases (Luirink et al, 2001;Kuhn et al, 2003;Yi and Dalbey, 2005).…”