cpSRP54 and FtsH cooperate thylakoid membrane associated proteostasis during de-etiolation inArabidopsis thaliana

Abstract

AbstractThe thylakoid membrane protein quality control, which requires the coordination of membrane protein translocation and degradation of unassembled proteins, determines the chloroplast development during de-etiolation. Despite numerous efforts, the regulation of this progress in land plants is largely unknown. Here, we reported the isolation and characterization ofpga4mutants with defects in chloroplast development during de-etiolation. Map-based cloning and complementation assay confirmed thatPGA4encodes the chloroplast signal recognition particle 54 kDa protein (cpSRP54). A heterogeneous LhcB2-GFP was generated as an indicative substrate for cpSRP54-mediated thylakoid translocation. LhcB2-GFP were not assembled into functional complexes, and degraded to a short form dLhcB2-GFP during de-etiolation, through an N-terminal degradation initiated on thylakoid membranes. Further biochemical and genetic evidences demonstrated that the degradation of LhcB2-GFP to dLhcB2-GFP was disrupted inpga4, andvar2mutants caused by mutations in the VAR2/AtFtsH2 subunit of thylakoid FtsH. The yeast two-hybrid assay showed that the N-terminus of LhcB2-GFP which was degraded consequently, interacts with the protease domain of VAR2/AtFtsH2 in yeasts. Moreover, the over-accumulated LhcB2-GFP inpga4andvar2, formed protein aggregates, which were insoluble in mild nonionic detergents. Genetically,cpSRP54is a new suppressor locus for the leaf variegation phenotype ofvar2. Those together demonstrated the coordination of cpSRP54 and thylakoid FtsH in maintenance of thylakoid membrane protein quality control during the assembly of photosynthetic complexes, and provided a trackable substrate and product for monitoring the cpSRP54-dependent protein translocation and the FtsH-dependent protein degradation.One Sentence SummaryWe revealed the coordination of cpSRP54 and FtsH in thylakoid membrane protein quality control, and provided a trackable marker for monitoring the activity of cpSRP54 and thylakoid FtsH protease.