The biological membrane is an efficient barrier against water-soluble substances. Solute transporters circumvent this membrane barrier by transporting water-soluble solutes across the membrane to the other sides. These transport proteins are thus required for all living organisms. Microorganisms, such as bacteria, effectively exploit solute transporters to acquire useful nutrients for growth or to expel substances that are inhibitory to their growth. Overall, there are distinct types of related solute transporters that are grouped into families or superfamilies. Of these various transporters, the major facilitator superfamily (MFS) represents a very large and constantly growing group and are driven by solute- and ion-gradients, making them passive and secondary active transporters, respectively. Members of the major facilitator superfamily transport an extreme variety of structurally different substrates such as antimicrobial agents, amino acids, sugars, intermediary metabolites, ions, and other small molecules. Importantly, bacteria, especially pathogenic ones, have evolved multidrug efflux pumps which belong to the major facilitator superfamily. Furthermore, members of this important superfamily share similar primary sequences in the form of highly conserved sequence motifs that confer useful functional properties during transport. The transporters of the superfamily also share similarities in secondary structures, such as possessing 12- or 14-membrane spanning α-helices and the more recently described 3-helix structure repeat element, known as the MFS fold. The three-dimensional structures of bacterial multidrug efflux pumps have been determined for only a few members of the superfamily, all drug pumps of which are surprisingly from Escherichia coli. This review briefly summarizes the structural properties of the bacterial multidrug efflux pumps of the major facilitator superfamily in a comparative manner and provides future directions for study.
more »
« less
This content will become publicly available on June 13, 2024
Passive endocytosis in model protocells
Semipermeable membranes are a key feature of all living organisms. While specialized membrane transporters in cells can import otherwise impermeable nutrients, the earliest cells would have lacked a mechanism to import nutrients rapidly under nutrient-rich circumstances. Using both experiments and simulations, we find that a process akin to passive endocytosis can be recreated in model primitive cells. Molecules that are too impermeable to be absorbed can be taken up in a matter of seconds in an endocytic vesicle. The internalized cargo can then be slowly released over hours, into the main lumen or putative cytoplasm. This work demonstrates a way by which primitive life could have broken the symmetry of passive permeation prior to the evolution of protein transporters.
more »
« less
- NSF-PAR ID:
- 10461722
- Date Published:
- Journal Name:
- Proceedings of the National Academy of Sciences
- Volume:
- 120
- Issue:
- 24
- ISSN:
- 0027-8424
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
more » « less
Summary Seed development largely depends on the long‐distance transport of sucrose from photosynthetically active source leaves to seed sinks. This source‐to‐sink carbon allocation occurs in the phloem and requires the loading of sucrose into the leaf phloem and, at the sink end, its import into the growing embryo. Both tasks are achieved through the function of
SUT sucrose transporters. In this study, we used vegetable peas (Pisum sativum L.), harvested for human consumption as immature seeds, as our model crop and simultaneously overexpressed the endogenousSUT 1SUT 1SUT 1SUT 1‐ and‐Pull approach for enhancing carbon transport is a successful strategy for improving seed yields and nutritional quality in legumes. -
more » « less
Transporters of the Nramp (Natural resistance-associated macrophage protein) family import divalent transition metal ions into cells of most organisms. By supporting metal homeostasis, Nramps prevent diseases and disorders related to metal insufficiency or overload. Previous studies revealed that Nramps take on a LeuT fold and identified the metal-binding site. We present high-resolution structures of
Deinococcus radiodurans (Dra)Nramp in three stable conformations of the transport cycle revealing that global conformational changes are supported by distinct coordination geometries of its physiological substrate, Mn2+, across conformations, and by conserved networks of polar residues lining the inner and outer gates. In addition, a high-resolution Cd2+-bound structure highlights differences in how Cd2+and Mn2+are coordinated by DraNramp. Complementary metal binding studies using isothermal titration calorimetry with a series of mutated DraNramp proteins indicate that the thermodynamic landscape for binding and transporting physiological metals like Mn2+is different and more robust to perturbation than for transporting the toxic Cd2+metal. Overall, the affinity measurements and high-resolution structural information on metal substrate binding provide a foundation for understanding the substrate selectivity of essential metal ion transporters like Nramps. -
more » « less
Abstract Membrane‐embedded transporters impart essential functions to cells as they mediate sensing and the uptake and extrusion of nutrients, waste products, and effector molecules. Promiscuous multidrug exporters are implicated in resistance to drugs and antibiotics and are highly relevant for microbial engineers who seek to enhance the tolerance of cell factory strains to hydrophobic bioproducts. Here, we report on the identification of small multidrug resistance (SMR) transporters in early‐branching anaerobic fungi (Neocallimastigomycetes). The SMR class of transporters is commonly found in bacteria but has not previously been reported in eukaryotes. In this study, we show that SMR transporters from anaerobic fungi can be produced heterologously in the model yeast
Saccharomyces cerevisiae , demonstrating the potential of these proteins as targets for further characterization. The discovery of these novel anaerobic fungal SMR transporters offers a promising path forward to enhance bioproduction from engineered microbial strains. -
more » « less
In contrast to an obsolete notion that erythrocytes, or red blood cells (
RBC s), play a passive and minor role in haemostasis and thrombosis, over the past decades there has been increasing evidence thatRBC s have biologically and clinically important functions in blood clotting and its disorders. This review summarizes the main mechanisms that underlie the involvement ofRBC s in haemostasis and thrombosisin vivo , such as rheological effects on blood viscosity and platelet margination, aggregation and deformability ofRBC s; direct adhesion and indirect biochemical interactions with endothelial cells and platelets. The ability of stored and pathologically alteredRBC s to generate thrombin through exposure of phosphatidylserine has been emphasized. The procoagulant and prothrombotic potential ofRBC ‐derived microparticles transfused with storedRBC s or formed in various pathological conditions associated with haemolysis has been described along with prothrombotic effects of free haemoglobin and haem. Binding of fibrinogen or fibrin toRBC s may influence their effects on fibrin network structure, clot mechanical properties and fibrinolytic resistance. Recent data on platelet‐driven clot contraction show thatRBC s compressed by platelets pulling on fibrin form a tightly packed array of polyhedral erythrocytes, or polyhedrocytes, which comprises a nearly impermeable barrier important for haemostasis and wound healing.RBC s may perform dual roles, both helping to stem bleeding but at the same time contributing to thrombosis in a variety of ways.
