Uncovering the Protein Translocon at the Chloroplast Inner Envelope Membrane

Chloroplast Translocon Revealed Protein translocation across biological membranes requires supramolecular complexes, called translocons. Chloroplasts require translocons in their double-envelope membranes to import thousands of nucleus-encoded proteins synthesized in the cytosol. However, the identity of the translocon at the inner envelope of the chloroplast (TIC) has long been a matter of debate; two proteins, Tic20 and Tic110, have been proposed to be central to protein translocation across the inner envelope membrane. Using transgenic Arabidopsis plants expressing a tagged form of Tic20, Kikuchi et al. (p. 571 ) report the isolation of a 1-megadalton complex composed of Tic56, Tic100, and Tic214 involved in protein translocation across the inner envelope. Thorough in vitro biochemical and in vivo genetic experimentation suggest that the isolated translocon contains both nuclear- and organellar-encoded components. Tic110 was not part of the isolated translocon.