Abstract
The GENOMES UNCOUPLED 1 (GUN1) gene has been reported to encode a chloroplast-localized pentatricopeptide-repeat protein, which acts to integrate multiple indicators of plastid developmental stage and altered plastid function, as part of chloroplast-to-nucleus retrograde communication. However, the molecular mechanisms underlying signal integration by GUN1 have remained elusive, up until the recent identification of a set of GUN1-interacting proteins, by co-immunoprecipitation and mass-spectrometric analyses, as well as protein–protein interaction assays. Here, we review the molecular functions of the different GUN1 partners and propose a major role for GUN1 as coordinator of chloroplast translation, protein import, and protein degradation. This regulatory role is implemented through proteins that, in most cases, are part of multimeric protein complexes and whose precise functions vary depending on their association states. Within this framework, GUN1 may act as a platform to promote specific functions by bringing the interacting enzymes into close proximity with their substrates, or may inhibit processes by sequestering particular pools of specific interactors. Furthermore, the interactions of GUN1 with enzymes of the tetrapyrrole biosynthesis (TPB) pathway support the involvement of tetrapyrroles as signaling molecules in retrograde communication.
Highlights
Upon illumination, proplastids differentiate into functional chloroplasts in developing photosynthetic tissues of cotyledons, leaves, and stems (Jarvis and López-Juez, 2013)
Precursor proteins carrying N-terminal transit peptides initially interact with two multiprotein complexes termed Translocon at the outer envelope membrane of chloroplasts (TOC) and Translocon at the inner envelope membrane of chloroplasts (TIC), which facilitate their active transport through the chloroplast envelope, powered by an ATP import motor, consisting of the stromal heatshock protein 93 (Hsp93), heat-shock protein 70 (Hsp70), and heat-shock protein 90 (Hsp90; Flores-Perez and Jarvis, 2013; Inoue et al, 2013; Shi and Theg, 2013a,b)
We describe the functional roles of the different GENOMES UNCOUPLED 1 (GUN1) protein partners and propose some testable hypotheses that should clarify the molecular role of GUN1 in chloroplast biogenesis and chloroplast protein homeostasis
Summary
Proplastids differentiate into functional chloroplasts in developing photosynthetic tissues of cotyledons, leaves, and stems (Jarvis and López-Juez, 2013). Six genome uncoupled (gun) mutants have been characterized in Arabidopsis thaliana that fail to repress transcription of the nuclear gene Lhcb1.2 after NF treatment, and are impaired in retrograde signaling (Susek et al, 1993; Mochizuki et al, 2001; Larkin et al, 2003; Koussevitzky et al, 2007; Adhikari et al, 2011; Woodson et al, 2011) Five of these genes, GUN2-6, were found to be involved in tetrapyrrole biosynthesis (TPB), whereas GUN1, which encodes a nucleoid-localized pentatricopeptide repeat protein (PPR), has been shown to have a role in PGE, and to act as an integrator of multiple retrograde signals, since gun mutants are unique in exhibiting a gun phenotype in response to both norfluorazon and lincomycin (Gray et al, 2003; Koussevitzky et al, 2007). The repertoire of nucleoid-associated proteins so far identified is quite extensive, the GUN1 protein is not listed in any of the chloroplast or nucleoid/pTAC proteomes published to date (for a review see Melonek et al, 2016), most probably because it accumulates in very small amounts at specific developmental stages or under
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