The chloroplast protein CPSAR1, dually localized in the stroma and the inner envelope membrane, is involved in thylakoid biogenesis

Abstract

SUMMARYThylakoid biogenesis is a crucial step for plant development involving the combined action of many cellular actors. CPSAR1 is shown here to be required for the normal organization of mature thylakoid stacks, and ultimately for embryo development. CPSAR1 is a chloroplast protein that has a dual localization in the stroma and the inner envelope membrane, according to microscopy studies and subfractionation analysis. CPSAR1 is close to the Obg nucleotide binding protein subfamily and displays GTPase activity, as demonstrated by in vitro assays. Disruption of the CPSAR1 gene via T-DNA insertion results in the arrest of embryo development. In addition, transmission electron microscopy analysis indicates that mutant embryos are unable to develop thylakoid membranes, and remain white. Unstacked membrane structures resembling single lamellae accumulate in the stroma, and do not assemble into mature thylakoid stacks. CPSAR1 RNA interference induces partially developed thylakoids leading to pale-green embryos. Altogether, the presented data demonstrate that CPSAR1 is a protein essential for the formation of normal thylakoid membranes, and suggest a possible involvement in the initiation of vesicles from the inner envelope membrane for the transfer of lipids to the thylakoids.

Genes co-expressed with CPSAR1 identified using ATTED-II

“…14 The genes found were mostly nuclear encoded, and their protein products were localized in the chloroplast (Table 1), in line with the organellar localization observed for CPSAR1. 12,13,15 The co-expressed proteins were highly expressed at the seedling, inflorescence and rosette leaf stages (data not shown), as previously shown for CPSAR1. 15 According to TMHMM (www.cbs.dtu.…”

“…12,13,15 The co-expressed proteins were highly expressed at the seedling, inflorescence and rosette leaf stages (data not shown), as previously shown for CPSAR1. 15 According to TMHMM (www.cbs.dtu. dk/services/TMHMM/) and SVMtm (ccb.imb.uq.edu.au/svmtm/), with the exception of EMB2458, none of the proteins coded by the co-expressed genes have transmembrane helices.…”

“…1). A total knockout of the CPSAR1 gene results in embryo lethality 12,13,15 but it has been suggested that this is linked with aberrant thylakoid biogenesis. 15 The remaining co-expressed proteins (RIF10, RAP and FAD) are involved in the regulation of plastidial isoprenoid pigments, plant disease resistance and metabolic processes.…”

“…html), both have a putative coiled-coil domain, like CPSAR1. 15 Proteins containing coiled-coil domains are important for protein or because of a change in CPSAR1 structure after GTP hydrolysis, in a similar manner to the cytosolic Sar1. 16 In addition, it is possible that a membrane protein cargo within the vesicle could interact with CPSAR1.…”

“…CPSAR1 does not have any transmembrane helices, but its dual localization in the stroma and the envelope membrane suggests that it can be either soluble or attached to the membrane. 15 Thus, attachment to the membrane could be a result of interaction with a membrane protein-protein interactions. 17 It is not unusual that GTP binding proteins work together (e.g., Toc33 and Toc159 1 ).…”