Determinants for stop-transfer and post-import pathways for protein targeting to the chloroplast inner envelope membrane.

摘要

Chloroplast biogenesis relies on the import of thousands of nuclear encoded proteins into the organelle and proper sorting to their sub-organellar compartment. The majority of nucleus-encoded chloroplast proteins are synthesized in the cytoplasm and imported into the organelle via the Toc-Tic translocation systems of the chloroplast envelope. In many cases, these proteins are further targeted to subcompartments of the organelle (e.g. the thylakoid membrane and lumen or inner envelope membrane) by additional targeting systems that function downstream of the import apparatus. The inner envelope membrane (IEM) plays key roles in controlling metabolite transport between the organelle and cytoplasm, and is the major site of lipid and membrane biogenesis within the organelle. In contrast to the protein import and thylakoid targeting systems, our knowledge of the pathways and molecular mechanisms of protein targeting and integration at the IEM are very limited. Previous reports have led to the conclusion that IEM proteins are transferred to the IEM during protein import via a stop-transfer mechanism. Recent studies have shown that at least two components of the Tic machinery (AtTic40 and AtTic110) are completely imported into the stroma and then re-inserted into the IEM in a post-import mechanism. This led me to investigate the mechanisms and pathways involved in the integration of chloroplast IEM proteins in more detail. I selected candidates (AtTic40 for post-import and IEP37 for stop-transfer) that are predicted to have only one membrane-spanning helix and adopt the same IEM topology to facilitate my analysis. My studies confirm the existence of both stop-transfer and post-import mechanisms of IEM protein targeting. Furthermore, I conclude that the IEP37 transmembrane domain (TMD) is a stop-transfer signal and is able of diverting AtTic40 to this pathway in the absence of AtTic40 IEM targeting information. Moreover, the IEP37 TMD also functions as a topology determinant. I also show that the AtTic40 targeting signals are context dependent, with evidence that in the absence of specific information in the appropriate context, the AtTic40 TMD behaves as a stop-transfer signal. This is an indication that the stop-transfer pathway is the default mechanism of protein insertion in the IEM.