Transit peptides play a major role in the preferential import of proteins into leucoplasts and chloroplasts.

The in vitro import characteristics of six different precursors of plastid proteins were assessed to determine differences in the protein import pathways of leucoplasts and chloroplasts. Five of these precursor proteins are destined to different subchloroplast sites, and one is a leucoplast stromal precursor protein. The results indicate that some of these precursors can be imported equally into both plastid types and others preferentially into one type of plastid versus the other. The ability of plastids to import different proteins correlates with the in vivo steady state levels of these proteins. Additional differences were also observed in the intraorganellar portion of the translocation pathway for two thylakoidal proteins. The differences in import characteristics were found to be predominantly governed by information in the transit peptides, since attachment of the various transit peptides to different plastid and foreign proteins demonstrated that the import behavior of the proteins is transferable with the transit sequence. These results indicate that the import mechanisms of leucoplasts and chloroplasts are sufficiently different such that the plastids respond differently to the information present in the transit peptides.