Genes co-expressed with CPSAR1 identified using ATTED-II

Identification of interacting proteins will help to investigate further the relationship between CPSAR1 and the vesicle transport system or the ribosomes. Thus, we adopted a bioinformatic approach, using the publicly available Arabidopsis thaliana trans-factor and cis-element prediction database, ATTED-II (http://atted.jp/), to identify putative protein interactors. The proteins directly linked to CPSAR1 were almost exclusively nucleus encoded and several were involved in protein synthesis of which three were thylakoid localized. The list of putative interacting proteins does not exclude any of the previous proposed actions of CPSAR1 but encourage more detailed examination of the role of CPSAR1.Key words: Arabidopsis, chloroplast, co-expression, protein cargo, vesicle transportChloroplast protein targeting has been intensively studied.^1–4 Transport of non-proteinaceous material, such as lipids, to the thylakoid has not been studied to the same extent, although several reports do exist.^5–8 Since thylakoids do not produce lipids themselves any lipids present must have been transported from the production site, i.e., the envelope.^9,10 Experimental data support the theory of a vesicular transport system and it has been predicted that several proteins resembling those important in the cytosolic vesicle transport system are chloroplast localized.¹¹ Recently we confirmed that one of these proteins, CPSAR1, is located in the chloroplast and has features similar to the cytosolic COPII-related Sar1 protein, i.e., it is found at the donor membrane and in the vesicle, but not at the acceptor membrane. In addition, other research groups^12,13 have also found that CPSAR1 is chloroplast localized but refer to it as atOBGL or atObgM, respectively, since CPSAR1 shares high sequence similarity with proteins belonging to the Obg-family. The roles of Obg proteins are very diverse, but a role closely connected to ribosomes has been suggested for CPSAR1.¹² CPSAR1 being part of a vesicle system does not exclude it from having a role in relation to ribosomes. The existence of a coiled-coil motif and a GTP binding domain, in combination with the suggested role as part of a vesicle transport system, makes it a highly attractive idea that CPSAR1 interacts with other proteins.