Molecular and Structural Characterization of the HMP-AB Gene Encoding a Pore-Forming Protein from a Clinical Isolate of Acinetobacter baumannii

Abstract

The major outer membrane protein of Acinetobacter baumannii is the heat-modifiable protein HMP-AB, a porin with a large pore size allowing the penetration of solutes having a molecular weight of up to approximately 800 Da. Cross-linking experiments with glutardialdehyde failed to show any cross-linking between the monomers, a fact that proves again that this porin protein functions as a monomeric porin. The specific activity of this porin was found to be similar to that of other monomeric porins. Tryptic digestion of the outer membrane yielded a 23-kDa fragment of the HMP-AB protein that was resistant to further trypsin treatment. This observation indicates that HMP-AB is assembled in the membrane in a manner similar to monomeric porins. Cloning of the HMP-AB gene revealed an open reading frame of 1038 bp encoding a protein of 346 amino acids and a calculated molecular mass of 35,636 Da. The amino acid sequence and composition were typical of gram-negative bacterial porins: a highly negative hydropathy index, absence of hydrophobic residue stretches, a slightly negative total charge, low instability index, high glycine content, and an absence of cysteine residues. Sequence comparison of HMP-AB with other outer membrane proteins revealed a clear homology with the monomeric outer membrane proteins, outer membrane protein A (OmpA) of Enterobacteria, and outer membrane protein F (OprF) of Pseudomonas sp. Secondary structure analysis indicated that HMP-AB has a 172-amino acid N-terminal domain that spans the outer membrane by eight amphiphilic beta strands and a C-terminal domain that apparently serves as an anchoring protein to the peptidoglycan layer. The results also indicate that HMP-AB belongs to the eight transmembrane beta-strand family of outer membrane proteins.

OmpA Is the Principal Nonspecific Slow Porin of Acinetobacter baumannii

“…Jyothisri et al (18) concluded that their OmpA Ab preparation had a pore-forming specific activity about equal to that of E. coli OmpF; however, the data in their paper show a specific activity about 100-fold lower than that of OmpF, and their conclusion was apparently due to a mistake in calculation. Gribun et al (15) concluded that their OmpA Ab preparation had an activity three to four times lower than that of E. coli OmF. This is still quite high, but we note that the same investigator in this team, Y. Nitzan, had earlier published data showing that OmpA Ab had specific activity 35-to 50-fold lower than that of E. coli OmpF (30).…”

“…OmpAb and HMP-AB were reported, however, to produce permeability comparable to or even higher than that of the classical trimeric porins of Escherichia coli, such as OmpF and OmpC (15,18). If this is correct, the presence of such porins does not explain the low permeability of the Acinetobacter OM.…”

“…The major protein of the Acinetobacter baumannii OM was named OmpAb (18), or HMP-AB (15), and belongs to the OmpAlike family, as sequence comparison revealed a clear homology with the monomeric OM protein A (OmpA) of Enterobacteriaceae and OM protein F (OprF) of Pseudomonas spp. OmpAb and HMP-AB were reported, however, to produce permeability comparable to or even higher than that of the classical trimeric porins of Escherichia coli, such as OmpF and OmpC (15,18).…”

Bacteroides Fragilis OmpA: Utility as a Live Vaccine Vector for Biodefense Agents

“…Acinetobacter baumannii [26], B. fragilis [14], and P. asaccharolytica [13]. Future work will include studying the role of the external loops of P. asaccharolytica…”

Recent Advances in the Immunopathogenesis of Acinetobacter baumannii Infection

“…This protein is homologous to OmpA proteins from Enterobacteria and outer membrane protein F (OprF) of Pseudomonas sp. (41). AbOmpA was reported to mediate cytotoxicity in human HEp-2 cells (32) and dendritic cells (33).…”