期刊
JOURNAL OF BIOLOGICAL CHEMISTRY
卷 281, 期 15, 页码 10548-10554出版社
AMER SOC BIOCHEMISTRY MOLECULAR BIOLOGY INC
DOI: 10.1074/jbc.M512687200
关键词
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The PSI-G subunit of photosystem I (PSI) is an 11-kDa membrane protein that plays an important role in electron transport between plastocyanin and PSI and is involved in the stability of the PSI complex. Within the complex, the PSI-G subunit is bound to PSI-B and is in contact with Lhca1. PSI-G has two transmembrane spans connected by a positively charged stromal loop. The loop is inaccessible to proteases, indicating a tightly bound location within the PSI complex. Here, we have studied the insertion mechanism and assembly of PSI-G. We show that the protein inserts into thylakoids by a direct or spontaneous pathway that does not involve the activities of any known chloroplast protein-targeting machinery. Surprisingly, the positively charged stromal loop region plays a major role in this process. Mutagenesis or deletions within this region almost invariably lead to a marked lowering of insertion efficiency, strongly indicating a critical role for the loop in the organization of the transmembrane regions prior to or during membrane insertion. Finally, we have examined the assembly of newly inserted PSI-G into the PSI complex, since very little is known of the assembly pathway for this large multimeric complex. Interestingly, we find that inserted PSI-G can be found within the full PSI complex within the import assay time frame after insertion into thylakoids, strongly suggesting that PSI-G normally associates at the end of the assembly process. This is consistent with its location on the periphery of the complex.
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