Translocase of Chloroplast 33, Chloroplastic (TOC33) Arabidopsis Thaliana Recombinant Protein

Function: 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. Binds GTP, GDP, XTP, but not ATP. Probably specialized in the import of nuclear encoded photosynthetic preproteins from the cytoplasm to the chloroplast, especially during early development stages. Ref.7 Ref.9 Ref.10 Ref.11 Ref.12 Ref.14 Ref.16 Ref.18 Ref.19 Ref.20

Cofactor: Binds 1 magnesium ion by subunit.

Subunit structure: Homodimer, heterodimer with TOC34 and TOC159, and monomer. The homodimerization and the dimerization with TOC159 require the binding of GTP on Arg-130, and an hypothetical coGAP factor. The dimeric form has a higher GTPase activity than the monomeric form. 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 at least with TOC75-3. Forms large complexes including TOC33, pPORA and OEP161 during pPORA import into plastids at the plastid envelope membrane. Ref.1 Ref.8 Ref.9 Ref.11 Ref.12 Ref.17 Ref.22 Ref.23 Ref.24

Subcellular location: Plastid › chloroplast outer membrane; Single-pass membrane protein. Note: May contain beta barrel transmembrane regions. Ref.1 Ref.12 Ref.13 Ref.15

Tissue specificity: Mostly expressed in seedlings and flowers, and, to a lower extent, in roots, stems, and leaves. Ref.1 Ref.14

Developmental stage: Mostly expressed in photosynthetic tissues undergoing rapid growth. Observed in cotyledons and vascular tissues of hypocotyls of young seedling. In roots, restricted to apical and lateral meristems, and vascular bundles. In stems, mostly detected in the upper part. Expressed in young and middle-aged leaves. In flowers, confined to sepals. Ref.1 Ref.14

Induction: Up-regulated by CIA2 in leaves. Induced in light but repressed in darkness. Ref.6 Ref.14

Post-translational modification: Phosphorylated by a kinase present in the outer envelope of chloroplast. When Ser-181 is phosphorylated, the binding to preprotein, GTP and GDP is inhibited, and thus, GTPase activity is repressed. Ref.9 Ref.18 Ref.20

Disruption phenotype: Plants exhibits a pale yellowish phenotype. Ref.1 Ref.7 Ref.14

Sequence similarities: Belongs to the TOC34 family.

Biophysicochemical propertiesKinetic parameters:KM=290 nM for GDP (at pH 7.4) Ref.9 Ref.21KM=5.7 µM for GTP (at pH 7.6 and 25 degrees Celsius)Vmax=1040 nmol/min/µg enzyme with GTP as substrate (at pH 7.4)