Heterotrimeric G-proteins mainly relay the information from G-protein-coupled receptors (GPCRs) on the plasma membrane to the inside of cells to regulate various biochemical functions. Depending on the targeted cell types, tissues and organs, these signals modulate diverse physiological functions. The basic schemes of heterotrimeric G-proteins have been outlined. In this review we briefly summarize what is known about the regulation, signaling and physiological functions of G-proteins. We then focus on a few less explored areas such as regulation of G-proteins by non-GPCRs, and the physiological functions of G-proteins that can not be easily explained by the known G-protein signaling pathways. There are new signaling pathways and physiological functions for G-proteins to be discovered and further interrogated. With the advancements in structural and computational biological techniques, we are closer to having a better understanding of how G-proteins are regulated, and the specificity of G-protein interactions with their regulators.

Background:The SENP/ULP family of SUMO proteases display different cleavage preference for SUMO isoforms. Results: Insights into the structural determinants for the preference of SENP6 and SENP7 for the SUMO2/3 isoform. Conclusion: A novel interface between SENP6 or SENP7 and SUMO determines the SUMO isoform preference. Significance: This may be the first time that the cleavage preference between SUMO1 and SUMO2/3 was swapped by single point mutagenesis.

Resistance to high concentrations of bile salts in the human intestinal tract is vital for the survival of enteric bacteria such as E scherichia coli. Although the tripartite AcrAB–TolC efflux system plays a significant role in this resistance, it is purported that other efflux pumps must also be involved. We provide evidence from a comprehensive suite of experiments performed at two different pH values (7.2 and 6.0) that reflect pH conditions that E . coli may encounter in human gut that MdtM, a single-component multidrug resistance transporter of the major facilitator superfamily, functions in bile salt resistance in E . coli by catalysing secondary active transport of bile salts out of the cell cytoplasm. Furthermore, assays performed on a chromosomal ΔacrB mutant transformed with multicopy plasmid encoding MdtM suggested a functional synergism between the single-component MdtM transporter and the tripartite AcrAB–TolC system that results in a multiplicative effect on resistance. Substrate binding experiments performed on purified MdtM demonstrated that the transporter binds to cholate and deoxycholate with micromolar affinity, and transport assays performed on inverted vesicles confirmed the capacity of MdtM to catalyse electrogenic bile salt/H+ antiport.

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Multidrug resistance arising from the activity of integral membrane transporter proteins presents a global public health threat. In bacteria such as Escherichia coli, transporter proteins belonging to the major facilitator superfamily make a considerable contribution to multidrug resistance by catalysing efflux of myriad structurally and chemically different antimicrobial compounds. Despite their clinical relevance, questions pertaining to mechanistic details of how these promiscuous proteins function remain outstanding, and the role(s) played by individual amino acid residues in recognition, binding and subsequent transport of different antimicrobial substrates by multidrug efflux members of the major facilitator superfamily requires illumination. Using in silico homology modelling, molecular docking and mutagenesis studies in combination with substrate binding and transport assays, we identified several amino acid residues that play important roles in antimicrobial substrate recognition, binding and transport by Escherichia coli MdtM, a representative multidrug efflux protein of the major facilitator superfamily. Furthermore, our studies suggested that 'aromatic clamps' formed by tyrosine and phenylalanine residues located within the substrate binding pocket of MdtM may be important for antimicrobial substrate recognition and transport by the protein. Such 'clamps' may be a structurally and functionally important feature of all major facilitator multidrug efflux proteins.The phenomenon of multidrug resistance -a serious global public health threat-arises principally from active efflux of drugs out of the cell cytoplasm by proteins that are integral membrane transporters. The 'transportome' of most bacteria contains numerous drug efflux proteins 1 , some of which are driven by energy released by ATP hydrolysis (the primary active transporters) and others by the energy stored in electrochemical gradients (the secondary active transporters).The majority of bacterial drug efflux proteins belong to the ubiquitous, large and diverse major facilitator superfamily (MFS) of secondary active transporters 2 . Although sequence similarity among the MFS is generally poor, all appear to follow the same structural template, their architecture consisting of either 12-or 14-transmembrane (TM) α -helices that are separated into six-or seven-helix bundle N-and C-terminal domains, respectively, related by a pseudo twofold symmetry and connected by a long cytoplasmic loop 3 . The N-and C-terminal halves of the transporter saddle a central substrate translocation pore, and both the N-and C-tails of the protein are located inside the cell. This structural arrangement implies that MFS proteins function via a single binding site, alternating access mechanism that is accompanied by a rocker switch-like movement of the two halves of the transporter, permitting access to the binding site(s) to swap between cytoplasm and periplasm 4 . The rocker switch mechanism supports at least three major conformational states during a dynamic transpor...