In vitro targeting of the Toc36 component of the chloroplast envelope protein import apparatus involves a complex set of information

Toc36 is a family of 44-kDa envelope polypeptides previously identified as components of the chloroplast protein import apparatus. Toc36 exists as multiple outer and inner envelope membrane forms. One member, Toc36B (formerly Bce44B), is targeted to the envelope without the typical maturation event. Targeting and assembly into the envelope is thus likely to involve a complex interplay of indigenous signals. These signals were examined by testing the effects of truncations and chimeric fusions on the targeting of Toc36B. The targeting ability of Toc36B appeared unaffected by carboxyl truncations of up to 80% of the protein, but was abolished by N-terminal deletions. The N-terminal 39 residues of Toc36B conferred the same targeting profile to mouse dihydrofolate reductase as that displayed by unaltered Toc36B. However, removal of 18 residues from the carboxyl end of the N-terminal 39-amino acid segment abolished targeting to the chloroplast. Additional information in the remaining Toc36B segment was also apparent based on the import results of chimeric fusions between the transit peptide of the small subunit of ribulose-1,5-bisphosphate carboxylase and Toc36B. The targeting of Toc36B to various destinations in the chloroplast envelope appears to be influenced by information from at least two segments of the protein. (C) 1999 Elsevier Science B.V. All rights reserved.