Mediates the insertion of proteins targeted to the outer membrane of chloroplasts. Required for the import of protein precursors into chloroplasts. Forms the voltage-dependent preprotein translocation channels (hydrophilic beta barrel) of the TOC complex in the chloroplastic outer membrane (By similarity). Part of the TOC core complex that includes a 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 precursors also interact with these complexes (By similarity). Transmembrane regions consist mainly of membrane-spanning sided beta-sheets, which are not predicted by sequence analysis tools. Belongs to the TOC75 family.