The Balance between Protein Synthesis and Degradation in Chloroplasts Determines Leaf Variegation inArabidopsis yellow variegatedMutants

Abstract

An Arabidopsis thaliana leaf-variegated mutant yellow variegated2 (var2) results from loss of FtsH2, a major component of the chloroplast FtsH complex. FtsH is an ATP-dependent metalloprotease in thylakoid membranes and degrades several chloroplastic proteins. To understand the role of proteolysis by FtsH and mechanisms leading to leaf variegation, we characterized the second-site recessive mutation fu-gaeri1 (fug1) that suppressed leaf variegation of var2. Map-based cloning and subsequent characterization of the FUG1 locus demonstrated that it encodes a protein homologous to prokaryotic translation initiation factor 2 (cpIF2) located in chloroplasts. We show evidence that cpIF2 indeed functions in chloroplast protein synthesis in vivo. Suppression of leaf variegation by fug1 is observed not only in var2 but also in var1 (lacking FtsH5) and var1 var2. Thus, suppression of leaf variegation caused by loss of FtsHs is most likely attributed to reduced protein synthesis in chloroplasts. This hypothesis was further supported by the observation that another viable mutation in chloroplast translation elongation factor G also suppresses leaf variegation in var2. We propose that the balance between protein synthesis and degradation is one of the determining factors leading to the variegated phenotype in Arabidopsis leaves.

Interplay Between N-Terminal Methionine Excision and FtsH Protease Is Essential for Normal Chloroplast Development and Function in Arabidopsis

“…Taken together, these results strongly suggest that, in the context of chloroplast biogenesis, the thylakoid FtsH protease complex operates downstream of PDF1B. Because PDF inhibition does not affect the rate of protein synthesis , the suppression effect associated with PDF inhibition shown here is therefore induced by a mechanism clearly uncoupled from protein synthesis inhibition Koussevitzky et al, 2007;Miura et al, 2007;Yu et al, 2008;Zhang et al, 2009;Liu et al, 2010).…”

“…At present, all suppressor genes of the ftsh-mediated variegated phenotype in Arabidopsis but one (see above) (Zhang et al, 2010) have been obtained by second-site suppressor screens Koussevitzky et al, 2007;Miura et al, 2007;Yu et al, 2008;Zhang et al, 2009;Liu et al, 2010). Unfortunately, the possible connection between the proteins encoded by the identified suppressor genes and the FtsH protease complex in the thylakoid remains unclear.…”

Thylakoid Development from Biogenesis to Senescence, and Ruminations on Regulation

“…FtsH and FUG1 are involved in thylakoid protein degradation and synthesis, respectively, suggesting that normal thylakoid development depends on maintaining balance between these processes. Reproduced from Figure 8A of Miura et al (2007).…”

Genetic suppression of plant development and chloroplast biogenesis via the Snowy Cotyledon 3 and Phytochrome B pathways

“…The latter seems to balance out the overproduction of proteins in sco3-1, which demonstrates the impairment in chloroplast development and thus results in greener seedlings. A similar observation has been made in revertants of the chloroplast development mutants variegated, where a reduction in chloroplast protein biosynthesis counterbalanced the deficiency in chloroplast protein degradation, resulting in the greening of the plants (Miura et al 2007). However, the most pronounced is the expression of genes in both gene pools that are involved in abiotic and biotic stress responses, indicating that continuous stress was present in the double mutant.…”