Toc159 is another GTP binding TOC subunit, like Toc34. Toc159 has three domains. At the N-terminal end is the A-domain, which is rich in acidic amino acids and takes up about half the protein length. The A-domain is often cleaved off, leaving an 86 kilodalton fragment called Toc86. In the middle is its GTP binding domain, which is very similar to the homologous GTP-binding domain in Toc34. At the C-terminal end is the hydrophilic M-domain, which anchors the protein to the outer chloroplast membrane.
Toc159 probably works a lot like Toc34, recognizing proteins in the cytosol using GTP. It can be regulated through phosphorylation, but by a different protein kinase than the one that phosphorylates Toc34. Its M-domain forms part of the tunnel that chloroplast preproteins travel through, and seems to provide the force that pushes preproteins through, using the energy from GTP.
Toc159 is not always found as part of the TOC complex—it has also been found dissolved in the cytosol. This suggests that it might act as a shuttle that finds chloroplast preproteins in the cytosol and carries them back to the TOC complex. There isn't a lot of direct evidence for this behavior though.
A family of Toc159 proteins, Toc159, Toc132, Toc120, and Toc90 have been found in Arabidopsis thaliana. They vary in the length of their A-domains, which is completely gone in Toc90. Toc132, Toc120, and Toc90 seem to have specialized functions in importing stuff like nonphotosynthetic preproteins, and can't replace Toc159.