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.

The interaction between light and gravity is critical in determining the final form of a plant. For example, the competing activities of gravitropism and phototropism can determine the final orientation of a stem or root. The results reported here indicate that, in addition to the previously described blue-light-dependent negative phototropic response in roots, roots of Arahidopsis thaliana (L.) Heynh. display a previously unknown red-light-dependent positive phototropic response. Both phototropic responses in roots are considerably weaker than the graviresponse, which often masks phototropic curvature. However, through the use of mutant strains with impaired gravitropism, we were able to identify a red-light-dependent positive phototropic response in Arabidopsis roots. The red-induced positive phototropic response is considerably weaker than the blue-light response and is barely detectable in plants with a normal gravitropic response.

Background: Proper development of plastids in embryo and seedling tissues is critical for plant development. During germination, plastids develop to perform many critical functions that are necessary to establish the seedling for further growth. A growing body of work has demonstrated that components of the plastid transcription and translation machinery must be present and functional to establish the organelle upon germination.

The three-legged or triskelion shape of clathrin is critical for the formation of polyhedral lattices around clathrincoated vesicles. Filamentous legs radiate from a common vertex, with amino acids 1550-1615 contributed by each leg to define the trimerization domain (Liu S-H, Wong ML, Craik CS, Brodsky FM. Cell 1995; 83: 257-267). Within this amino acid stretch there are 3 cysteines at positions 1565, 1569 and 1573 which are completely conserved in higher mammals from humans to C. elegans. The cysteine-toserine mutation at position 1573 was observed to have the largest impact on clathrin structure and self-assembly. We have also found that Cysteine 1528 located near the boundary between the proximal region and trimerization domain mediated the formation of nonproductive clathrin aggregates when bound light chain subunits were removed. However, when light chains were added back, the ability of this cysteine to form disulfide bridges between individual clathrin molecules was blocked, suggesting bound light chain interacted with Cysteine 1528 to prevent aggregation. This new information serves to map the orientation of the light chain subunit in the vicinity of the trimerization domain and supports previous models that indicate involvement of the trimerization domain in LC binding ( The ability of clathrin to self-assemble is tightly linked to its molecular pinwheel shape, which facilitates the formation of clathrin-coated vesicles (CCVs) involved in carrying out receptor-mediated endocytosis at both the plasma membrane and the trans-Golgi network (TGN) (1,2). The triskelionshaped clathrin molecule comprises three identical heavy chains, each with a bound light chain subunit (2) responsible for regulating clathrin assembly (3,4). The heavy chains are joined together along a stretch near their C-terminal ends to form the trimerization domain. This domain establishes the specific 3-fold symmetry between projecting legs and also serves to gather the N-terminal domains of free clathrin molecules (triskelions) as they integrate into the growing clathrin lattice during the normal course of clathrin self-assembly (5). Recent docking studies of atomic resolution clathrin sub-domains into the 21 Å cryo-EM structure of the intact clathrin cage revealed the orientation between legs emanating from each lattice vertex was invariant (6), suggesting the trimerization domain was highly stabilized.In an effort to understand the molecular basis for trimerization domain stability, we set out to probe the role of specific cysteines (1565, 1569 and 1573) closely clustered together within this region of the clathrin molecule. To facilitate these studies, recombinant hub, which is the central region of the clathrin triskelion (3), was used instead of the full-length protein. Recombinant hubs (1074-1675) are trimeric structures with the ability to self-assemble into lattice arrays closely resembling those made by whole clathrin, but cannot form completed cages because they lack the N-terminal and distal domains required to develop curvatu...

Our results indicate that GGPS1 is a key gene in the chlorophyll biosynthetic pathway.