Drug Resistance in Bacteria , Fungi , Malaria , and Cancer

The biological membrane covers all living cells and provides an effective barrier against the passage of biologically important water-soluble solutes. This natural passage barrier is essentially overcome with the use of integral membrane proteins known as solute transporters. These transport systems translocate solutes across the membrane such as in the case of bacterial drug and multidrug resistance efflux pumps. One of the largest groups of transporters is referred to as the major facilitator superfamily. This group contains secondary active transporters such as symporters and antiporters and passive transporters such as uniporters. The transporters within the major facilitator superfamily share conserved structures and primary amino acid sequences. In particular, several highly conserved amino acid sequence motifs have been discovered and studied extensively, providing substantial evidence for their critical functional roles in the transport of solutes across the membrane. 1 Importance of Solute Transport in Living Organisms All known living cells are surrounded by a biological membrane that provides an effective barrier against the passage of aqueous-based solutes and ions. Living cells, however, must be able to acquire helpful substances while also extruding harmful ones. Biological membranes solve this barrier problem by using integral membrane proteins that selectively catalyze the acquisition and efflux of helpful P. Kakarla • R. KC • U. Shrestha • I. Ranaweera • M.M. Mukherjee • T.M. Willmon • A.J. Hernandez • S.R. Barr • M.F. Varela (*) Department of Biology, Eastern New Mexico University, Station 33, Portales, NM 88130, USA e-mail: Manuel.Varela@enmu.edu © Springer International Publishing Switzerland 2017 G. Arora et al. (eds.), Drug Resistance in Bacteria, Fungi, Malaria, and Cancer, DOI 10.1007/978-3-319-48683-3_4 111 Manuel.Varela@enmu.edu and harmful water-soluble molecules, respectively. Therefore, integral membrane solute transporters are important for all life on Earth (Broome-Smith 1999). When solute transporters are defective, medical disease may occur such as those seen in glucose–galactose malabsorption (Wright et al. 2002), Fanconi–Bickel syndrome (Santer et al. 2002), and De Vivo disease (De Vivo et al. 1991), which are genetic diseases involving impaired transport of glucose across the membranes of cells and develop from inheritable mutations which occur in the genes that encode monosaccharide sugar transporters, thus impairing the uptake of monosaccharides into cells. Bacteria use solute transporters to efflux multiple antimicrobial agents, often causing loss of chemotherapeutic efficacy during treatment of infectious diseases (Chopra 1992; Kumar and Varela 2013; Li et al. 2015). Solute transporters that multidrug-resistant bacteria use to efflux antimicrobial agents can be grouped into several protein families, such as the ABC (ATP-binding cassette) transporters (Higgins 1992), the resistance-nodulation-cell division (RND) superfamily (Tseng et al. 1999), the small multidrug resistance (SMR) superfamily (Chung and Saier 2001), the multidrug and toxic compound extrusion (MATE) superfamily (Kuroda and Tsuchiya 2009; Kumar et al. 2013), and the major facilitator superfamily (MFS) (Paulsen et al. 1996b; Pao et al. 1998; Saier et al. 1999; Kumar and Varela 2012; Andersen et al. 2015). This review will focus on the antimicrobial agent efflux pumps of the MFS and especially MFS pumps of known structures. Particular attention will be paid to studies which have involved amino acid residues that belong to highly conserved sequence motifs A and C of the MFS (Griffith et al. 1992; Marger and Saier 1993). 2 Acquisition of Helpful Nutrients and Efflux of Harmful Solutes Substances are routinely transported across biological membranes of living organisms. These substances include an extremely diverse range of water-soluble solutes such as amino acids, Krebs cycle intermediates, sugars, nucleic acids, neurotransmitters, antimicrobial agents, and other small molecules (Henderson et al. 1998). Nutrient uptake via solute transport is a crucial process in which living cells acquire and accumulate molecules from the external environment in order to support metabolism, cell growth, and cell maintenance. On the other hand, living organisms must be able to efflux toxic substances from the inside of their cells into the extracellular milieu in order to maintain growth and survival. Living bacterial cells, for example, have developed integral membrane proteins to facilitate efflux of toxic molecules, a trait that confers antimicrobial resistance (Kumar and Varela 2013). 112 P. Kakarla et al. Manuel.Varela@enmu.edu 3 Types of Solute Transporter Systems Transport systems play important roles in the cellular uptake of helpful molecules such as nutrients, ions, and small molecules and in the exit of harmful or inhibitory molecules. Cellular entry and exit of solutes can occur in two general ways: passive and active transport. Passive transport entails the movement of small molecules across the membrane and does not require biological energy to do so (Mitchell 1967; West and Mitchell 1972). Active transport systems move solutes across the membrane against their own solute concentration gradients (i.e., from low to high concentrations), using integral membrane proteins, called pumps or active transporters. This type of solute transport is referred to as active because of the energy required to conduct transport across the biological membrane (Henderson 1991; Hediger 1994). 3.1 Passive Solute Transport In passive transport systems, solutes are translocated across the membrane from a side of the membrane with relatively high solute concentration toward the side with relatively low solute concentration, i.e., down the solute concentration gradient (Hediger 1994). The passive solute transport systems generally do not require the expenditure of biological energy. Transport systems use integral membrane carriers to catalyze solute uniport, a facilitative diffusion process that enables a single molecular species to be transported down their concentration gradients (Henderson 1991; Saier 2000). 3.1.1 Facilitated Diffusion Facilitated diffusion refers to solute transport involving poreor carrier-forming molecules. In this process, solute reversibly binds to a solute-specific carrier protein that resides integral to the membrane. The complex of solute and carrier oscillates between the innerand outer-facing surfaces of the biological membrane, thus causing binding and release of the solute to the other side of the same membrane (Henderson 1991). A special class of integral membrane proteins, called porins, form large nonspecific water-filled channels within the outer membrane to allow the acquisition of nutrients from the periplasm of Gram-negative bacteria. These channels are also associated with the efflux of the waste products (Nikaido 1994). Many so-called classical porins examined so far are OmpC, OmpF, and PhoE from Escherichia coli (Nikaido and Vaara 1985; Nikaido 1992). These porins exist as closely associated trimeric complexes that cannot be dissociated even with sodium dodecyl sulfate (SDS), unless heated denatured beforehand (Reid et al. 1988). Functional Roles of Highly Conserved Amino Acid Sequence Motifs A and C in. . . 113 Manuel.Varela@enmu.edu These porins show preferences on the basis of solute size and charge. In the case of charge, OmpC and OmpF prefer cations slightly more compared to anions, and PhoE prefers anions. OmpF allows translocation of relatively larger solutes compared to OmpC, showing preferences according to the size of the solute (Nikaido 2003). 3.2 Active Transporter Systems Two main energy-requiring solute transporter systems, i.e., primary active transport (energized by hydrolysis of ATP) and secondary active transport (energized by ion gradients), are used to efflux biomolecules from bacteria (Mitchell 1966, 1972, 1991, 2011; Harold 2001). Among the dozens of primary and secondary active transporter families, two such superfamilies in particular occur in a ubiquitous manner across all taxonomic categories of living organisms. These systems include a superfamily called the ATP-binding cassette (ABC) transporters and another group called the major facilitator superfamily (MFS) of transporters (Pao et al. 1998; Saier et al. 1999; Davidson and Maloney 2007; Law et al. 2008). 3.2.1 Primary Active Solute Transporters In primary active transport, the free energy required for solute transport against the electrochemical gradient is provided by the very protein performing the transport. They do so by the hydrolysis of adenosine triphosphate (ATP) (Tarling et al. 2013). Often referred to ABC transporters (Higgins 1992), these primary active transporters represent a large group of integral membrane proteins that couple the transport of a substrate like amino acids, ions, sugars, lipids, and drugs across the membrane (Chang 2003) to the hydrolysis of the phosphate bond between the γand the β-phosphate of ATP (ter Beek et al. 2014). It includes both importers and exporters (Locher 2009), bringing nutrients and other molecules into cells or exporting toxins, drugs, and lipids across membranes (Rees et al. 2009). To attain export, ABC transporters use four types of subunits called domains, two transmembrane domains (TMDs) plus two nucleotide binding domains (NBDs). TMDs provide specificity and form the binding sites for ligand, and NBDs undertake ATP hydrolysis to accomplish the translocation across the membrane of its bound solute. However, import requires an additional periplasmic binding domain (PBP) (Linton 2007; Procko et al. 2009). A conformational change in the TMDs occurs once substrate binds, followed by transmission to the NBDs to initiate ATP hydrolysis (Higgins 2001). ABC transporters adopt at least two conformations, i.e., the cis-side or the trans-side. The binding site for the solute