The Significance of Protein Maturation by Plastidic Type I Signal Peptidase 1 for Thylakoid Development in Arabidopsis Chloroplasts

Thylakoids are the chloroplast internal membrane systems that house light-harvesting and electron transport reactions. Despite the important functions and well-studied constituents of thylakoids, the molecular mechanism of their development remains largely elusive. A recent genetic study has demonstrated that plastidic type I signal peptidase 1 (Plsp1) is vital for proper thylakoid development in Arabidopsis (Arabidopsis thaliana) chloroplasts. Plsp1 was also shown to be necessary for processing of an envelope protein, Toc75, and a thylakoid lumenal protein, OE33; however, the relevance of the protein maturation in both of the two distinct subcompartments for proper chloroplast development remained unknown. Here, we conducted an extensive analysis of the plsp1-null mutant to address the significance of lumenal protein maturation in thylakoid development. Plastids that lack Plsp1 were found to accumulate vesicles of variable sizes in the stroma. Analyses of the mutant plastids revealed that the lack of Plsp1 causes a reduction in accumulation of thylakoid proteins and that Plsp1 is involved in maturation of two additional lumenal proteins, OE23 and plastocyanin. Further immunoblotting and electron microscopy immunolocalization studies showed that OE33 associates with the stromal vesicles of the mutant plastids. Finally, we used a genetic complementation system to demonstrate that accumulation of improperly processed forms of Toc75 in the plastid envelope does not disrupt normal plant development. These results suggest that proper maturation of lumenal proteins may be a key process for correct assembly of thylakoids.