Toc34 is an integral protein in the outer chloroplast membrane that's anchored into it by its hydrophobic C-terminal tail. Most of the protein, however, including its large guanosine triphosphate (GTP)-binding domain projects out into the stroma.
Toc34s job is to catch some chloroplast preproteins in the cytosol and hand them off to the rest of the TOC complex. When GTP, an energy molecule similar to ATP attaches to Toc34, the protein becomes much more able to bind to many chloroplast preproteins in the cytosol. The chloroplast preproteins presence causes Toc34 to break GTP into guanosine diphosphate (GDP) and inorganic phosphate. This loss of GTP makes the Toc34 protein release the chloroplast preprotein, handing it off to the next TOC protein. Toc34 then releases the depleted GDP molecule, probably with the help of an unknown GDP exchange factor. A domain of Toc159 might be the exchange factor that carry out the GDP removal. The Toc34 protein can then take up another molecule of GTP and begin the cycle again.
Toc34 can be turned off through phosphorylation. A protein kinase drifting around on the outer chloroplast membrane can use ATP to add a phosphate group to the Toc34 protein, preventing it from being able to receive another GTP molecule, inhibiting the protein's activity. This might provide a way to regulate protein import into chloroplasts.
Arabidopsis thaliana has two homologous proteins, AtToc33 and AtToc34 (The At stands for Arabidopsis thaliana), which are each about 60% identical in amino acid sequence to Toc34 in peas (called psToc34). AtToc33 is the most common in Arabidopsis, and it is the functional analogue of Toc34 because it can be turned off by phosphorylation. AtToc34 on the other hand cannot be phosphorylated.