Porins and small-molecule translocation across the outer membrane of Gram-negative bacteria

Abstract

Gram-negative bacteria and their complex cell envelope comprising an outer and inner membrane are an important and attractive system for studying the translocation of small molecules across biological membranes. In the outer membrane of Enterobacteriaceae, trimeric porins control the cellular penetration of small molecules, including nutrients and antibacterial agents. The synergistic action between relatively slow porin-mediated passive uptake across the outer membrane and active efflux transporters in the inner membrane creates a permeability barrier that reinforces the enzymatic modification barrier, which efficiently reduces the intracellular concentrations of small molecules and contributes to the emergence of antibiotic resistance. In this review, we discuss recent advances in our understanding of the molecular and functional roles of classic porins in small molecule translocation in Enterobacteriaceae and consider the crucial role of porins in antibiotic resistance. Commented [w1]: Is this specification necessary here?, in my opinion it deviates, better to put later… Commented [JP2]: Editor request... porins represent the preferred route for the entry of β-lactams, including cephalosporins, penicillins and carbapenems 14-16. The clinical relevance of membrane-associated mechanisms (MAMs) of resistance (i.e. porin defects and/or overexpression of multidrug efflux pumps) has been well established for these antibiotics. The Influx and Efflux rates control the internal concentration of antibiotics and represent the first lane (mechanical barrier) protecting the bacterial cells against therapeutic treatment 1-3,6. Consequently, studies on bacterial porins are receiving a renewed interest due to their key role in the bacterial susceptibility towards clinically used antibiotics. In combination with the expression of antibiotic-modifying enzymes expressed in the periplasm (e.g. β-lactamases), porins play a key role in β-lactam resistance 4,17. In this review, we discuss recent advances in our understanding of the molecular and functional roles of classic porins in antibiotic translocation in Enterobacteriaceae. We explore structural aspects and the insights gained into permeation and the pore translocation process, the regulation of porin expression as well as the role of porins in the emergence of antibiotic susceptibility. Enterobacterial general porins Structural aspects The crystal structures of a general porin from Rhodobacter capsulatus 18 , the OmpF and PhoE porins from E. coli 19 and other E. coli OmpF structures including mutants 20,21 were the first to be solved. Only a limited number of other enterobacterial porin structures have been reported, i.e. E. coli OmpC, K. pneumoniae OmpK36 and Salmonella typhi OmpF 22-24. The lack of data has hindered attempts to relate structure to function. Recently, the structures of two porins from P. stuartii as well as the structures of the OmpF and OmpC orthologs of K. pneumoniae, E. aerogenes and E. cloacae have been reported 12,25,26. Another recent study reported th...

Evolutionary selection of a 19-stranded mitochondrial β-barrel scaffold bears structural and functional significance

“…Bacterial β-barrels are presented with hostile extracellular environments and a distinctly asymmetric lipopolysaccharide–phospholipid membrane ( 46 ). These unusually rigid scaffolds accommodate prolonged turnover times through significantly high kinetic stability ( 13 , 47 ). In contrast, VDACs require a metastable scaffold to achieve structural dynamicity necessary for its homo- and heterooligomerization and diverse functions ranging from flux of various metabolites, signaling in the asymmetric mitochondrial bilayer membrane, to its dynamic interactome with metabolic, cytosolic, and apoptotic proteins ( 5 , 25 , 48 ).…”

“…These mitochondrial β-barrel outer membrane proteins (mOMPs) constitute ∼1% of the total human proteome), are found exclusively in the OMM, and were believed to have endosymbiotic origins ( 9 ). The first structures of isoform 1 of human and mouse VDACs resolved to atomic resolution in 2008 ( 10 – 12 ) revealed the most unexpected finding that unlike the even-stranded porins pervasive in bacteria ( 13 ), VDACs adopt a unique β-barrel structure with an odd number of β-strands, i.e. 19 β-strands.…”

Structure and Stoichiometry of the Ton Molecular Motor

“…In addition to the cytoplasmic membrane, Gram-negative bacteria possess an additional outer membrane that acts as an efficient barrier against the environment [1]. While small molecules are able to diffuse from the exterior of the cell into the periplasmic space through specialized porins, some nutrients only exist in low concentration in the extracellular medium and hence, need active transport for their uptake [2,3]. However, both the outer membrane and the periplasmic space are depleted in chemical energy sources such as nucleotide hydrolysis, therefore, the energy must come from other sources [4].…”

The crystal structure of the TonB-dependent transporter YncD reveals a positively charged substrate binding site

“…The outer membrane of Gram-negative bacteria represents a formidable permeability barrier to hydrophilic molecules larger than ~600 Da (1). Hydrophilic molecules smaller than this diffusion limit cross the outer membrane through proteins called porins, which can be either promiscuous or substrate-specific (2). To import molecules that are too large for diffusion through outer-membrane porins, bacteria employ a diverse family of membrane proteins termed TonB-dependent transporters (TBDTs) (3).…”