Topology and neighbor analysis of the photosynthetic reaction center from Rhodopseudomonas sphaeroides

Isoform composition of connexin channels determines selectivity among second messengers and uncharged molecules

Intercellular connexin channels (gap junction channels) have long been thought to mediate molecular signaling between cells, but the nature of the signaling has been unclear. This study shows that connexin channels from native tissue have selective permeabilities, partially based on pore diameter, that discriminate among cytoplasmic second messenger molecules. Permeability was assessed by measurement of selective loss/retention of tracers from liposomes containing reconstituted connexin channels. The tracers employed were tritiated cyclic nucleotides and a series of oligomaltosaccharides derivatized with a small uncharged fluorescent moiety. The data define different size cut-off limits for permeability through homomeric connexin-32 channels and through heteromeric connexin-32/connexin-26 channels. Connexin-26 contributes to a narrowed pore. Both cAMP and cGMP were permeable through the homomeric connexin-32 channels. cAMP was permeable through only a fraction of the heteromeric channels. Surprisingly, cGMP was permeable through a substantially greater fraction of the heteromeric channels than was cAMP. The data suggest that isoform stoichiometry and/or arrangement within a connexin channel determines whether cyclic nucleotides can permeate, and which ones. This is the first evidence for connexin-specific selectivity among biological signaling molecules.

Evidence for an association of the early light-inducible protein (ELIP) of pea with photosystem II

The precursor to the nuclear-coded 17 kDa early light-inducible protein (ELIP) of pea has been transported into isolated intact chloroplasts. The location of the mature protein in the thylakoid membranes was investigated after using cleavable crosslinkers such as DSP and SAND in conjunction with immunofractionation methods and by application of mild detergent fractionation. We show that ELIP is integrated into the membranes via the unstacked stroma thylakoids. After isolation of protein complexes by solubilization of membranes with Triton X-100 and sucrose density-gradient centrifugation the crosslinked ELIP comigrates with the PS II core complex. Using SAND we identified ELIP as a 41-51 kDa crosslinked product while with DSP four products of 80 kDa, 70 kDa, 50-42 kDa and 23-21 kDa were found. The immunoprecipitation data suggested that the D1-protein of the PS II complex is one of the ELIP partners in crosslinked products.

Investigation of the spatial relationships between photosystem 2 polypeptides by reversible crosslinking and diagonal electrophoresis

Nearest neighbour relationships within the LHC2-PS2 complex were investigated by using the reversible crosslinking agent dithiobis(succinimidyl propionate) (DSP). This was accomplished by treating PS2-enriched membranes, prepared from chloroplasts of Pisum sativum, with the crosslinker followed by diagonal electrophoresis of the solubilised polypeptides.Analysis of the off-diagonal spot patterns produced by crosslinker cleavage and second dimension electrophoresis was made on the basis of: staining with Coomassie blue or silver, labelling with [(35)S]-methionine, and sensitivity to 1 M NaCl washing. It was concluded that LHC2 polypeptides crosslinked with several components of the PS2 complex and that the extrinsic polypeptides associated with water oxidation, having approximate molecular weights of 16 and 23 kDa, crosslink to form homodimers. The latter finding suggests that there may be more than one copy of each of these polypeptides per PS2 complex.

Cloning and expression of the Rhodobacter sphaeroides reaction center H gene

The Rhodobacter sphaeroides structural gene (puhA) for the reaction center H polypeptide has been identified and cloned by using restriction fragements specific for the analogous Rhodobacter capsulatus gene as a heterologous hybridization probe. The presence of puhA on a 1.45-kilobase BamHI restriction fragment was confirmed by partial DNA sequence analysis and by the synthesis of an immunoreactive Mr-28,000 reaction center H polypeptide in an R. sphaeroides coupled transcription-translation system. Approximately 450 base pairs of DNA upstream of the puhA gene were sufficient for expression of this protein in vitro. Northern RNA-DNA blot analysis with an internal puhA-specific probe identified at least two, apparently monocistronic, transcripts present at different cellular levels under physiological conditions known to affect the cellular content of both reaction center complexes and photosynthetic membrane. Northern blot analysis with specific upstream restriction fragment probes revealed that the 1,400-nucleotide puhA-specific mRNA had a 5' terminus upstream of the 1,130-nucleotide transcript. Both puhA-specific mRNA and immunoreactive reaction center H protein were detectable in chemoheterotrophically grown cells which lacked detectable bacteriochlorophyll and photosynthetic membrane.

In vitro biosynthesis and membrane association of photosynthetic reaction center subunits from Rhodopseudomonas sphaeroides

The reaction center of Rhodopseudomonas sphaeroides is an integral membrane protein complex responsible for primary photochemical charge separation in photosynthesis. We report the synthesis of two of the three subunits of the photosynthetic reaction center using a DNA-directed in vitro transcription-translation system prepared from R. sphaeroides. The in vitro-synthesized polypeptides, as resolved by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, had apparent Mrs of 24,000 and 21,000 and were shown to be synthesized in equimolar amounts. This corresponds precisely to the in vivo reaction center subunits M and L, respectively. The in vitro-synthesized polypeptides were immunoprecipitated with antibody prepared against whole native reaction centers. In addition, the identity of the in vitro-synthesized polypeptides as L and M was verified by comparing the protease digestion products of in vivo- with in vitro-synthesized reaction center subunits. Both of the in vitro-synthesized polypeptides were also found to partition with the particulate material in the transcription-translation system and to associate with added membranes.