Influence of preformed alpha-helix and alpha-helix induction on the activity of cationic antimicrobial peptides.

One prominent class of cationic antibacterial peptides comprises the alpha-helical class, which is unstructured in free solution but folds into an amphipathic alpha-helix upon insertion into the membranes of target cells. To investigate the importance of alpha-helicity and its induction on interaction with membranes, a series of peptides was constructed based on a hybrid of moth cecropin (amino acids 1-8) and bee melittin (amino acids 1-18) peptides. The new peptides were predicted to have a high tendency to form alpha-helices or to have preformed alpha-helices by virtue of construction of a lactam bridge between glutamate and lysine side-chains at positions i and i + 4 at various locations along the primary sequence. In two examples where the use of lactam bridge constraints induced and stabilized alpha-helical structure in benign (aqueous buffer) and/or hydrophobic medium, there was a decrease in antibacterial activity relative to the linear counterparts. Thus the preformation of alpha-helix in solution was not necessarily beneficial to antimicrobial activity. In the one case where the lactam bridge did result in increased antibacterial activity (lower minimal inhibitory concentration values) it did not increase alpha-helical content in benign or hydrophobic medium. Broadly speaking, good activity of the peptides against Pseudomonas aeruginosa correlated best (r2 = 0.88) with a helican parameter which was calculated as the induction of alpha-helix in a membrane-mimicking environment divided by the alpha-helix formation under benign conditions. Interestingly, the activity of the lactam bridge peptide constructs correlated in part with alterations in bacterial outer or cytoplasmic membrane permeability.

Determinants of recombinant production of antimicrobial cationic peptides and creation of peptide variants in bacteria.

Cationic peptides possessing antibacterial activity are virtually ubiquitous in nature, and offer exciting prospects as new therapeutic agents. We had previously demonstrated that such peptides could be produced by fusion protein technology in bacteria and several carrier proteins had been tested as fusion partners including glutathione-S-transferase, S. aureus protein A, IgG binding protein and P. aeruginosa outer membrane protein OprF. However these fusion partners, while successfully employed in peptide expression, were not optimized for high level production of cationic peptides (Piers, K., Brow, M. L., and Hancock, R. E. W. 1993, Gene 137, 7-13). In this paper we took advantage of a small replication protein RepA from E. coli and used its truncated version to construct fusion partners. The minimal elements required for high level expression of cationic peptide were defined as a DNA sequence encoding a fusion protein comprising, from the N-terminus, a 68 amino acid carrier region, an anionic prepro domain, a single methionine and the peptide of interest. The 68 amino acid carrier region was a block of three polypeptides consisting of a truncated RepA, a synthetic cellulose binding domain and a hexa histidine domain. The improved system showed high level expression and simplified downstream purification. The active peptide could be yielded by CNBr cleavage of the fusion protein. This novel vector was used to express three classes of cationic peptides including the alpha-helical peptide CEMA, the looped peptide bactenecin and the extended peptide indolicidin. In addition, mutagenesis of the peptide gene to produce peptide variants of CEMA and indolicidin using the improved vector system was shown to be successful.

Effect of the PufQ protein on early steps in the pathway of bacteriochlorophyll biosynthesis in Rhodobacter capsulatus.

The addition in trans of the pufQ gene to a strain of Rhodobacter capsulatus from which the entire puf operon had been deleted, increased its ability to synthesize coproporphyrinogen from both delta-aminolevulinic acid and porphobilinogen. Studies at the enzyme level indicated that the conversion of porphobilinogen to uroporphyrinogen III had about a 2-fold higher level of activity in the anaerobically-grown pufQ-containing strain. This increase in activity over the puf-deletion strain appeared to occur during transitions from aerobic to semiaerobic growth conditions. These results indicated that the PufQ protein may exert a stimulatory effect quite early in the pathway of bacteriochlorophyll biosynthesis.

Identification of the PufQ protein in membranes of Rhodobacter capsulatus.

The PufQ protein has been detected in vivo for the first time by Western blot (immunoblot) analyses of the chromatophore membranes of Rhodobacter capsulatus. The PufQ protein was not visible in Western blots of membranes of a mutant (delta RC6) lacking the puf operon but appeared in membranes of the same mutant to which the pufQ gene had been added in trans. It was also detected in elevated amounts in a mutant (CB1200) defective in two bch genes and unable, therefore, to make bacteriochlorophyll. The extremely hydrophobic nature of the PufQ protein was also apparent in these studies since it was not extracted from chromatophores by 3% (wt/vol) n-octyl-beta-D-glucopyranoside, a procedure which solubilized the reaction center and light-harvesting complexes. During adaptation of R. capsulatus from aerobic to semiaerobic growth conditions (during which time the synthesis of bacteriochlorophyll was induced), the PufQ protein was observed to increase to the level of detection in the developing chromatophore fraction approximately 3 h after the start of the adaptation. The enzyme, S-adenosyl-L-methionine:magnesium protoporphyrin methyltransferase, also increased in amount in the developing chromatophore fraction but was present in a cell membrane fraction at the start of the adaptation as well.

Association of protochlorophyllide with the PufQ protein of Rhodobacter capsulatus.

The PufQ protein, prepared by the recombinant expression of the pufQ gene of Rhodobacter capsulatus, has been reconstituted into liposomes in the presence of the bacteriochlorophyll precursor, protochlorophyllide. The liposomes were separated from liposomes free of the PufQ protein by sucrose density gradient ultracentrifugation and analyzed for their content of protochlorophyllide, protein, and phospholipid. The results indicated a 3.5 times higher level of association of protochlorophyllide with the PufQ protein-containing liposomes than with liposomes containing either the hydrophobic protein, apolipoprotein A-I, or the water-soluble maltose-binding protein. Only the association of the protochlorophyllide with the PufQ protein corresponded to a small but reproducible shift in its fluorescence emission maximum to a lower wavelength, consistent with a change in its environment.

Recombinant expression of the pufQ gene of Rhodobacter capsulatus.

Genetic studies have shown that the expression of the pufQ gene is required for normal levels of bacteriochlorophyll biosynthesis in Rhodobacter capsulatus. Yet, the exact function of the pufQ gene is unknown, and a pufQ gene product has never been isolated. We describe the recombinant overexpression of pufQ in Escherichia coli, as well as the purification and characterization of its gene product, the 74-amino-acid PufQ protein. Site-directed mutagenesis was used to facilitate the cloning of the pufQ gene into various expression vector systems of E. coli, including pKK223-3, pLcII-FX, and pMal-c. Although high levels of pufQ transcription were evident from constructs of all three vectors, high levels of protein expression were apparent only in the pMal-c system. In vector pMal-c, the recombinant PufQ protein is expressed as a fusion with an amino-terminal maltose-binding domain. After affinity purification on an amylose column, full-length PufQ protein was released from the fusion protein by limited proteolysis with the enzyme factor Xa. The PufQ protein demonstrated a strong tendency to associate with phospholipid vesicles, consistent with the view that it is an integral membrane protein. The PufQ protein was subsequently purified by high-performance liquid chromatography and identified by amino-terminal sequence analysis. A possible role for the PufQ protein in the transport of bacteriochlorophyll biosynthetic intermediates is discussed.