State transitions are an important photosynthetic short-term response that allows energy distribution balancing between photosystems I (PSI) and II (PSII). In plants when PSII is preferentially excited compared with PSI (State II), part of the major light-harvesting complex LHCII migrates to PSI to form a PSI-LHCII supercomplex. So far, little is known about this complex, mainly due to purification problems. Here, a stable PSI-LHCII supercomplex is purified from Arabidopsis thaliana and maize (Zea mays) plants. It is demonstrated that LHCIIs loosely bound to PSII in State I are the trimers mainly involved in state transitions and become strongly bound to PSI in State II. Specific Lhcb1-3 isoforms are differently represented in the mobile LHCII compared with S and M trimers. Fluorescence analyses indicate that excitation energy migration from mobile LHCII to PSI is rapid and efficient, and the quantum yield of photochemical conversion of PSI-LHCII is substantially unaffected with respect to PSI, despite a sizable increase of the antenna size. An updated PSI-LHCII structural model suggests that the lowenergy chlorophylls 611 and 612 in LHCII interact with the chlorophyll 11145 at the interface of PSI. In contrast with the common opinion, we suggest that the mobile pool of LHCII may be considered an intimate part of the PSI antenna system that is displaced to PSII in State I.

We previously have shown that Nicotiana sylvestris cytoplasmic male sterile (CMS) mutants I and II present large mtDNA deletions and that the NAD7 subunit of complex I (the main dehydrogenase of the mitochondrial respiratory chain) is absent in CMS I. Here, we show that, despite a large difference in size in the mtDNA deletion, CMS I and II display similar alterations. Both have an impaired development from germination to f lowering, with partial male sterility that becomes complete under low light. Besides NAD7, two other complex I subunits are missing (NAD9 and the nucleus-encoded, 38-kDa subunit), identified on twodimensional patterns of mitochondrial proteins. Mitochondria isolated from CMS leaves showed altered respiration. Although their succinate oxidation through complex II was close to that of the wild type, oxidation of glycine, a priority substrate of plant mitochondria, was significantly reduced. The remaining activity was much less sensitive to rotenone, indicating the breakdown of Complex I activity. Oxidation of exogenous NADH (coupled to proton gradient generation and partly sensitive to rotenone) was strongly increased. These results suggest respiratory compensation mechanisms involving additional NADH dehydrogenases to complex I. Finally, the capacity of the cyanide-resistant alternative oxidase pathway was enhanced in CMS, and higher amounts of enzyme were evidenced by immunodetection.

Plant plasma membrane intrinsic proteins (PIPs) are aquaporins that facilitate the passive movement of water and small neutral solutes through biological membranes. Here, we report that post-Golgi trafficking of PIP2;7 in Arabidopsis thaliana involves specific interactions with two syntaxin proteins, namely, the Qc-SNARE SYP61 and the Qa-SNARE SYP121, that the proper delivery of PIP2;7 to the plasma membrane depends on the activity of the two SNAREs, and that the SNAREs colocalize and physically interact. These findings are indicative of an important role for SYP61 and SYP121, possibly forming a SNARE complex. Our data support a model in which direct interactions between specific SNARE proteins and PIP aquaporins modulate their post-Golgi trafficking and thus contribute to the fine-tuning of the water permeability of the plasma membrane.

The SNQ2 gene of Saccharomyces cerevisiae, which encodes an ATP binding cassette protein responsible for resistance to the mutagen 4-nitroquinoline oxide, is regulated by the DNA-binding proteins PDR1 and PDR3. In a plasma membrane-enriched fraction from a pdr1 mutant, the SNQ2 protein is found in the 160-kDa over-expressed band, together with PDR5. The SNQ2 protein was solubilized with n-dodecyl beta-D-maltoside from the plasma membranes of a PDR5-deleted strain and separated from the PMA1 H(+/-)ATPase by sucrose gradient centrifugation. The enzyme shows a nucleoside triphosphatase activity that differs biochemically from that of PDR5 (Decottignies, A., Kolaczkowski, M., Balzi, E., and Goffeau, A. (1994) J. Biol. Chem. 269, 12797-12803) and is sensitive to vanadate, erythrosine B, and Triton X-100 but not to oligomycin, which inhibits the PDR5 activity only. Disruption of both PDR5 and SNQ2 in a pdr1 mutant decreases the cell growth rate and reveals the presence of at least two other ATP binding cassette proteins in the 160-kDa overexpressed band that have been identified by amino-terminal microsequencing.