Peculiar properties of the PsaF photosystem I protein from the green alga Chlamydomonas reinhardtii: presequence independent import of the PsaF protein into both chloroplasts and mitochondria

It has previously been shown that presequences of nuclear-encoded chloroplast proteins from the green alga Chlamydomonas reinhardtii contain a region that may form an amphiphilic α-helix, a structure characteristic of mitochondrial presequences. We have tested two precursors of chloroplast proteins (the PsaF and PsaK photosystem I subunits) from C. reinhardtii for the ability to be imported into spinach leaf mitochondria in vitro. Both precursors bound to spinach mitochondria. The PsaF protein was converted into a protease-protected form with high efficiency in a membrane potential-dependent manner, indicating that the protein had been imported, whereas the PsaK protein was not protease protected. The protease protection of PsaF was not inhibited by a synthetic peptide derived from the presequence of the N. plumbaginifolia mitochondrial F1β subunit. Furthermore, if the presequence of PsaF was truncated or deleted by in vitro mutagenesis, the protein was still protease-protected with approximately the same efficiency as the full-length precursor. These results indicate that PsaF can be imported by spinach mitochondria in a presequence-independent manner. However, even in the absence of the presequence, this process was membrane potential-dependent. Interestingly, the presequence-truncated PsaF proteins were also protease-protected upon incubation with C. reinhardtii chloroplasts. Our results indicate that the C. reinhardtii chloroplast PsaF protein has peculiar properties and may be imported not only into chloroplasts but also into higher-plant mitochondria. This finding indicates that additional control mechanisms in the cytosol that are independent of the presequence are required to achieve sorting between chloroplasts and mitochondria in vivo.