Cell-cycle-specific fluctuation in cytoplasmic membrane composition in aerobically grown Rhodospirillum rubrum

The role of the cell cycle in the cellular uptake of folate-modified poly(L-amino acid) micelles in a cell population

Nanoparticles are widely recognized as a vehicle for tumor-targeted therapies. There are many factors that can influence the uptake of nanoparticles, such as the size of the nanoparticles, and/or their shape, elasticity, surface charge and even the cell cycle phase. However, the influence of the cell cycle on the active targeting of a drug delivery system has been unknown until now. In this study, we initially investigated the folate receptor α (FR-α) expression in different phases of HeLa cells by flow cytometric and immunocytochemical methods. The results obtained showed that FR-α expression was cell cycle-dependent, i.e. the S cells' folate receptor expression was the highest as the cell progressed through its cycle. Then, we used folate modified poly(L-amino acid) micelles (FA-PM) as an example to investigate the influence of the cell cycle on the active targeting drug delivery system. The results obtained indicated that the uptake of FA-PM by cells was influenced by the cell cycle phase, and the S cells took up the greatest number of folate conjugated nanoparticles. Our findings suggest that future studies on ligand-mediated active targeting preparations should consider the cell cycle, especially when this system is used for a cell cycle-specific drug.

A new system for heterologous expression of membrane proteins: Rhodospirillum rubrum

Heterologous expression of membrane proteins has met with only limited success. This work presents a new host/vector system for the production of heterologous membrane proteins based on a mutant of the facultatively phototrophic bacterium Rhodospirillum rubrum. Under certain growth conditions, R. rubrum forms an intracytoplasmic membrane (ICM) that houses the photosynthetic apparatus, the structural proteins of which are encoded by puhA and pufBALM. The mutant R. rubrum H2, which was constructed by allelic exchange deleting puhA and pufBALM, does not form ICM. This strain was used as a host for a plasmid expressing the Pseudomonas aeruginosa membrane protein MscL from the Rhodobacter capsulatus puc promoter. ICM was formed in the H2 strain producing MscL but not in the vector control strain. These results suggest that a heterologous membrane protein stimulates ICM formation in R. rubrum and indicate that the capacity to form an ICM that can accommodate heterologous proteins makes R. rubrum a host that will be useful for membrane protein production. P. aeruginosa MscL, which forms inclusion bodies when produced in Escherichia coli, was expressed in R. rubrum H2 and purified from membranes with a yield of 22.8-23.4 mg/L culture (5.53-5.60 mg/g cell paste). Additionally Streptomyces lividans KcsA and P. aeruginosa CycB were produced and purified from R. rubrum H2 with yields of 13.7-14.4 mg/L culture (2.19-2.55 mg/g cell paste) and 6.6-7.4 mg/L culture (1.1-1.2mg/g cell paste), respectively.

Role of the H protein in assembly of the photochemical reaction center and intracytoplasmic membrane in Rhodospirillum rubrum

Rhodospirillum rubrum is a model for the study of membrane formation. Under conditions of oxygen limitation, this facultatively phototrophic bacterium forms an intracytoplasmic membrane that houses the photochemical apparatus. This apparatus consists of two pigment-protein complexes, the light-harvesting antenna (LH) and photochemical reaction center (RC). The proteins of the photochemical components are encoded by the puf operon (LHalpha, LHbeta, RC-L, and RC-M) and by puhA (RC-H). R. rubrum puf interposon mutants do not form intracytoplasmic membranes and are phototrophically incompetent. The puh region was cloned, and DNA sequence determination identified open reading frames bchL and bchM and part of bchH; bchHLM encode enzymes of bacteriochlorophyll biosynthesis. A puhA/G115 interposon mutant was constructed and found to be incapable of phototrophic growth and impaired in intracytoplasmic membrane formation. Comparison of properties of the wild-type and the mutated and complemented strains suggests a model for membrane protein assembly. This model proposes that RC-H is required as a foundation protein for assembly of the RC and highly developed intracytoplasmic membrane. In complemented strains, expression of puh occurred under semiaerobic conditions, thus providing the basis for the development of an expression vector. The puhA gene alone was sufficient to restore phototrophic growth provided that recombination occurred.

Chromium(VI) reductase activity is associated with the cytoplasmic membrane of anaerobically grown Shewanella putrefaciens MR-1

Shewanella putrefaciens MR-1 can reduce a diverse array of compounds under anaerobic conditions, including manganese and iron oxides, fumarate, nitrate, and many other compounds. These reductive processes are apparently linked to a complex electron transport system. Chromium (Cr) is a toxic and mutagenic metal and bacteria could potentially be utilized to immobilize Cr by reducing the soluble and bioavailable state, Cr(VI), to the insoluble and less bioavailable state, Cr(III). Formate-dependent Cr(VI) reductase activity was detected in anaerobically grown cells of S. putrefaciens MR-1, with highest specific activity in the cytoplasmic membrane. Both formate and NADH served as electron donors for Cr(VI) reductase, whereas L-lactate or NADPH did not support any activity. The addition of 10 micromol l(-1) FMN markedly stimulated formate-dependent Cr(VI) reductase, and the activity was almost completely inhibited by diphenyliodonium chloride, an inhibitor of flavoproteins. Cr(VI) reductase activity was also inhibited by p-chloromercuriphenylsulphonate, azide, 2-heptyl-4-hydroxyquinolone-N-oxide, and antimycin A, suggesting involvement of a multi-component electron transport chain which could include cytochromes and quinones. Cr(V) was detected by electron paramagnetic resonance (EPR) spectroscopy, suggesting a one-electron reduction as the first step.

Isolation and sequence of omcA, a gene encoding a decaheme outer membrane cytochrome c of Shewanella putrefaciens MR-1, and detection of omcA homologs in other strains of S. putrefaciens

The sequence of the omcA gene, which encodes a decaheme cytochrome c that is localized to the outer membrane (OM) of Shewanella putrefaciens MR-1, was determined. The 2202 bp nucleotide sequence of omcA encodes for 734 amino acids with a predicted molecular protein mass of 78.6 kDa. Comparison with the amino-terminal sequence of the mature protein suggests the presence of a hydrophobic leader sequence which is cleaved during translocation of the protein to the OM. This leader sequence has a lipoprotein consensus sequence for signal peptidase II at the cleavage site. The predicted mature protein is comprised of 708 amino acids with a predicted molecular mass of 75.8 kDa, but the addition of ten covalently attached heme c groups and covalent lipid modification to the amino-terminal cysteine increases the predicted mass to 82.7 kDa. This is consistent with its apparent mass of 83 kDa in SDS-PAGE gels. The predicted amino acid sequence for the OmcA protein shows no significant homology to known proteins. A RNA of approx. 2300 bases that hybridizes to the omcA gene was detected in anaerobically grown MR-1 cells. The size of this transcript is similar to the coding region of the omcA gene, suggesting that it is not part of a multicistronic operon. Similar to MR-1, four other strains of S. putrefaciens were all found to localize a majority of their membrane-bound cytochromes to the OM when grown under anaerobic conditions, and all contained an OM cytochrome of similar size to OmcA. In two of these strains, MR-4 and MR-8, a homolog of omcA was identified by RT-PCR and Southern blotting using primers and probes specific for omcA of MR-1. Western blot analysis using a polyclonal antibody to OmcA was similarly positive in strains MR-4 and MR-8. Partial nucleotide sequence analysis of these homologs demonstrated 74-77% predicted amino acid homology with OmcA of MR-1. In contrast, strains MR-30 and MR-42 tested negative for omcA homologs by Southern and Northern blots, RT-PCR, and Western blots.

Outer membrane cytochromes of Shewanella putrefaciens MR-1: spectral analysis, and purification of the 83-kDa c-type cytochrome

The metal-reducing bacterium Shewanella putrefaciens MR-1 is known to localize a majority of its membrane-bound cytochromes to its outer membrane when grown under anaerobic conditions. In this study, pyridine hemochrome spectra confirmed that these outer membrane cytochromes are c-type, and electrophoretic data demonstrated the presence of four distinct outer membrane cytochromes, with apparent molecular masses of 150, 83, 65, and 53 kDa. Fourth-order derivative analysis of 77 K spectra of the outer membrane revealed four spectrally distinct c-type hemes, with peaks at 545.4, 548.0, 550.6, and 552.6 nm. Outer membrane cytochromes in the reduced state were rapidly re-oxidized by oxidized iron and manganese, which have previously been shown to serve as electron acceptors for anaerobic respiration in this bacterium. The 83-kDa outer membrane cytochrome was purified and a specific polyclonal antibody was generated against this protein. Western blot analysis demonstrated that the vast majority of this protein was localized to the outer membrane and an intermediate density membrane fraction of similar composition. Its levels, but not its subcellular distribution, were somewhat influenced by the electron acceptor used to support anaerobic growth, with levels higher in fumarate-grown cells relative to iron(III)- or trimethylamine N-oxide-grown cells. Its specific content in cells grown under aerobic conditions was only 14% of that of fumarate-grown cells, suggesting that a switch to anaerobic conditions significantly increases the de novo synthesis of this outer membrane cytochrome.

Cloning and sequence of cymA, a gene encoding a tetraheme cytochrome c required for reduction of iron(III), fumarate, and nitrate by Shewanella putrefaciens MR-1

The cymA gene, which encodes a tetraheme cytochrome c, was cloned from Shewanella putrefaciens MR-1. This gene complemented a mutant which had a TnphoA insertion in cymA and which was deficient in the respiratory reduction of iron(III), nitrate, fumarate, and manganese(IV). The 561-bp nucleotide sequence of cymA encodes a protein of 187 amino acids with a predicted molecular mass of 20.8 kDa. No N-terminal signal sequence was readily apparent; consistent with this, a cytochrome with a size of 21 kDa was detected in the wild type but was absent in the insertional mutant. The cymA gene is transcribed into an mRNA; the major transcript was approximately 790 bases, suggesting that it is not part of a multicistronic operon. This RNA transcript was not detected in the cymA mutant. The CymA protein was found in the cytoplasmic membrane and soluble fraction of MR-1, and it shares partial amino acid sequence homology with multiheme c-type cytochromes from other bacteria. These cytochromes are ostensibly involved in the transfer of electrons from the cytoplasmic membrane to acceptors in the periplasm. The localization of the fumarate and iron(III) reductases to the periplasm and outer membrane of MR-1, respectively, suggests the possibility of a similar electron transfer role for CymA.

Identification and partial sequence of the BchA gene of Rhodospirillum rubrum

The DNA sequence was determined for a region upstream of the puf operon of Rhodospirillum rubrum. A partial ORF was identified. The DNA sequence and the inferred amino acid sequences were aligned with those of bchA of Rhodobacter capsulatus and other phototrophic bacteria. Based on this alignment and genetic evidence, this ORF was identified as R. rubrum bchA, which encodes an enzyme involved in bacteriochlorophyll biosynthesis. An additional ORF was identified in the intergenic region between bchA and pufB. Sequences possibly having a role in puf expression were identified.

Localization of cytochromes to the outer membrane of anaerobically grown Shewanella putrefaciens MR-1

In gram-negative bacteria, numerous cell functions, including respiration-linked electron transport, have been ascribed to the cytoplasmic membrane. Gram-negative bacteria which use solid substrates (e.g., oxidized manganese or iron) as terminal electron acceptors for anaerobic respiration are presented with a unique problem: they must somehow establish an electron transport link across the outer membrane between large particulate metal oxides and the electron transport chain in the cytoplasmic membrane. When the metal-reducing bacterium Shewanella putrefaciens MR-1 is grown under anaerobic conditions and membrane fractions are purified from cells lysed by an EDTA-lysozyme-polyoxyethylene cetyl ether (Brij 58) protocol, approximately 80% of its membrane-bound cytochromes are localized in its outer membrane. These outer membrane cytochromes could not be dislodged by treatment with chaotropic agents or by increased concentrations of the nonionic detergent Brij 58, suggesting that they are integral membrane proteins. Cytochrome distribution in cells lysed by a French press protocol confirm the localization of cytochromes to the outer membrane of anaerobically grown cells. This novel cytochrome distribution could play a key role in the anaerobic respiratory capabilities of this bacterium, especially in its ability to mediate manganese and iron reduction.

Construction, characterization, and complementation of Rhodospirillum rubrum puf region mutants

Rhodospirillum rubrum is a facultatively phototrophic bacterium that, under certain growth conditions, forms an intracytoplasmic chromatophore membrane (ICM) housing the photochemical apparatus. The puf operon of R. rubrum encodes protein subunits of the photochemical reaction center and the B880 light-harvesting antenna complex. Mutant strains of R. rubrum were constructed by interposon mutagenesis through which a kanamycin resistance gene cartridge was inserted into restriction sites and in place of restriction fragments of the puf region. Southern blot analysis demonstrated that the defective copies of puf sequences had replaced their normal chromosomal counterparts through homologous recombination. The phenotypes of the mutant strains were evaluated on the basis of puf gene expression, spectral analysis, pigment content of membranes, and electron-microscopic examination of thin sections of cells grown under semi-aerobic and dark anaerobic conditions. Alterations of the puf region affect phototrophic competence and the formation of the ICM. The latter result implies an obligatory role for puf gene products in ICM formation in R. rubrum. One mutant with a deletion in puf structural genes was complemented in trans to the wild-type phenotype. Other mutants could be restored to the wild-type phenotype only by recombination.