GTPase involved in protein precursor import into chloroplasts. Seems to recognize chloroplast-destined precursor proteins and regulate their presentation to the translocation channel through GTP hydrolysis. Probably specialized in the import of nuclear encoded photosynthetic preproteins from the cytoplasm to the chloroplast. Binds 1 Mg(2+) ion by subunit. Homodimer (By similarity). Part of the TOC core complex that includes 1 protein for the specific recognition of transit peptides surrounded by a ring composed of four proteins forming translocation channels, and four to five GTP-binding proteins providing energy. This core complex can interact with components of the TIC complex to form a larger import complex. Chloroplastic protein precursor such as prSS (precursor of the RuBisCO small subunit) interacts with these complexes. The TOC complex contains a specific subset of polar lipids such as digalactosyldiacylglyceride (DGDG), phosphatidylcholine (PC) and phosphatidylglycerol (PG). Interacts with TOC33 and TOC75. Cycles between the cytoplasm and chloroplast, probably as a soluble preprotein receptor. The anchoring to the chloroplast outer membrane required the GTPase activity and GDP. May contain beta barrel transmembrane regions (Probable). A number of isoforms are produced. According to EST sequences. Expressed in seedlings, leaves, flowers, and roots. By light conditions. Belongs to the TRAFAC class TrmE-Era-EngA-EngB-Septin-like GTPase superfamily. AIG1/Toc34/Toc159-like paraseptin GTPase family. TOC159 subfamily.