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As sessile organisms, plants constantly monitor environmental cues and respond appropriately to modulate their growth and development. Membrane transporters act as gatekeepers of the cell regulating both the inflow of useful materials as well as exudation of harmful substances. Members of the multidrug and toxic compound extrusion (MATE) family of transporters are ubiquitously present in almost all forms of life including prokaryotes and eukaryotes. In bacteria, MATE proteins were originally characterized as efflux transporters conferring drug resistance. There are 58 MATE transporters in Arabidopsis thaliana, which are also known as DETOXIFICATION (DTX) proteins. In plants, these integral membrane proteins are involved in a diverse array of functions, encompassing secondary metabolite transport, xenobiotic detoxification, aluminium tolerance, and disease resistance. MATE proteins also regulate overall plant development by controlling phytohormone transport, tip growth processes, and senescence. While most of the functional characterizations of MATE proteins have been reported in Arabidopsis, recent reports suggest that their diverse roles extend to numerous other plant species. The wide array of functions exhibited by MATE proteins highlight their multitasking ability. In this review, we integrate information related to structure and functions of MATE transporters in plants. Since these transporters are central to mechanisms that allow plants to adapt to abiotic and biotic stresses, their study can potentially contribute to improving stress tolerance under changing climatic conditions.

The temperature dependence of the 31P chemical shifts of the ribodinucleoside monophosphates, ApA, GpC, CpC, UpU, and ApU, of the deoxyribonucleic acids, d-ApT, TpT, d-ApA, and d-pApT, and of the homopolyribonucleic acids poly(G), poly(U), poly(A) is shown to provide information on the helix-coli transition in nucleic acids. The base stacked, helical structure with a gauche,gauche phosphate ester torsional conformation is 0.2-0.6 ppm upfield from the random coil conformation. In contrast, the 31P chemical shifts of dimethyl and diethyl phosphate do not change significantly with temperature. These results support our earlier hypothesis that 31P shifts are sensitive probes of torsional conformations with phosphate esters in gauche,gauche conformations having 31P shifts upfield from nongauche conformations.

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A "coupling" of RO-XTOR (X = C or P) bond angle to rotation about the O-X bond in dimethoxymethane (DMM) and dimethyl phosphate (DMP) has been established through ab initio and semiempirical CNDO molecular orbital calculations. Analysis of structural data (x ray and electron diffraction) on related molecules supports this conclusion. Specifically, rotation about the O-X bond from a gauche to a trans conformation results in ca. a 5°reduction in the O-X-O bond angle. Thus, the CNDO optimized O-C-O bond angle in DMM is 112°for the g,g conformation, 107°for the g,t conformation, and 102°for the t,t conformation (ab initio geometries are comparable). A similar change is observed for DMP. This bond angle distortion may be useful in identifying important interactions between lone electron pairs and bonds, which are suggested to be responsible for the observed bond angle variation. Reversal of the relative energies for the g,g and g,t conformations is possible without bond angle optimization in the calculations and establishes the importance of optimization of this geometric parameter in molecular orbital torsional barrier calculations.

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The GAUSSIAN 70 program has been modified at Princeton University to permit automatic conversion from 4-310 to 5-21G for lithium and beryllium. Documentation has been submitted to the Quantum Chemistry Program Exchange.(9) W. Abstract: Angle strain and torsional strain energies were calculated using the CNDO/2 LCAO-MO method for various geometries of dimethyl phosphate monoanion and compared with energies for 2',3'-cyclic ribose phosphate and 3',5'-cyclic ribose phosphate. While the calculations fail to identify the source of the strain energy in the 3',5'-cyclic six-membered ring nucleotides, they do indicate that a significant portion of the high heat of hydrolysis of the five-membered ring, 2',3'-cyclic nucleotides is associated with relief of torsional strain and that preferred torsional conformations of acyclic esters are strongly coupled to the RO-P-OR bond angles. This coupling of ester, 0 -P -0 bond angles, and torsional angles is also demonstrated by CNDO calculations on various geometries of trimethyl phosphate. Eclipsing of one of the phosphate ester bonds reduces the bond angle between the esterified oxygen atoms by ca. 5O and eclipsing of both ester bonds further reduces the bond angle by another S o . These predictions of the CNDO calculations are shown to be supported by x-ray crystallographic structures of cyclic and acyclic, monoanionic, and neutral phosphate esters.