Transport-deficient mutants

Esko, J. D., Elgavtsh, A., Prasthofer, T Taylor, W, J1., and VV inke, J. L. (1986). Sulfate transport-deficient mutants of Chinese hairtster ovary cells, /. Biot. Chem. 261, 15725-15733. 

Carter NS, Drew ME, Sanchez M et al. Cloning of a novel inosine-guanosine transporter gene from Leishmanla donovani by functional rescue of a transport-deficient mutant. J Biol Chem 2000 275 20935-20941. 

Initially, the knowledge of transport systems in bacteria was obtained from studies with whole cells. The existence of primary and secondary transport systems was demonstrated and some information about the stoichiometry, specificity and kinetic constants of the transport proteins was obtained. In particular, the isolation of transport-deficient mutants established the functional role of specific transport systems. However, the information about molecular aspects of the transport processes which can be obtained from studies with intact cells is limited. The translocation process of the solute under study may be affected by the two additional cell-envelope... 

Mayer R, Kartenbeck J, Buchler M, Jedlitschky G, Leier I, Keppler D. Expression of the MRP gene-encoded conjugate export pump in liver and its selective absence from the canalicular membrane in transport- deficient mutant hepatocytes. J Cell Biol 1995 131 137-150. 

Masereeuw R, Notenboom S, Smeets PH, et al. Impaired renal secretion of substrates for the multidrug resistance protein 2 in mutant transport-deficient (TR-) rats. J Am Soc Nephrol 2003 14 2741-2749. 

These alleles, accounting for over half of all mutant LDL receptor alleles, specify transport-deficient receptor precursors that fail to move with normal rates from the endoplasmic reticulum to and through the Golgi apparatus and on to the cell surface. While some mutations merely attenuate processing, most of these mutations are complete in that transport from the endoplasmic reticulum fails and the mutant receptors never reach the cell surface. 

As described above, desiccation stimulates ABA synthesis and there are high concentrations of ABA in water-stressed leaves. This relation leads to the proposal that ABA mediates the response to drought. Indeed, most ABA deficient mutants display an excessive water loss due to defects in stomatal regulation which leads to an increased tendency to wilt [28,44,99]. Characterization of ABA deficient tomato plants showed a direct relationship between stomatal closure and ABA. The stomata of these plants were unresponsive to water stress but could be closed by applying ABA. ABA synthesized in water-stressed roots may act as a chemical signal to induce stomatal closure before any desiccation of the leaves. When the roots are submitted to stress they will produce ABA that is transported to the aerial parts of the plant and induces stomatal closure to prevent evaporation. This eould be a meehanism to optimize the plant s water use under restricted availability [19]. 

The evidence that (- )-shikimic acid plays a central role in aromatic biosynthesis was obtained by Davis with a variety of nutritionally deficient mutants of Escherichia coli. In one group of mutants with a multiple requirement for L-tyrosine, L-phenylalanine, L-tryptophan and p-aminobenzoic acid and a partial requirement for p-hydroxybenzoic acid, (—)-shikimic acid substituted for all the aromatic compounds. The quintuple requirement for aromatic compounds which these mutants displayed arises from the fact that, besides furnishing a metabolic route to the three aromatic a-amino acids, the shikimate pathway also provides in micro-organisms a means of synthesis of other essential metabolites, and in particular, the various isoprenoid quinones involved in electron transport and the folic acid group of co-enzymes. The biosynthesis of both of these groups of compounds is discussed below. In addition the biosynthesis of a range of structurally diverse metabolites, which are derived from intermediates and occasionally end-products of the pathway, is outlined. These metabolites are restricted to certain types of organism and their function, if any, is in the majority of cases obscure. 

Ishida, N., Ito, M., Yoshioka, S., Sun-Wada, G.H., and Kawakita, M. Functional expression of human golgi CMP-sialic acid transporter in the Golgi complex of a transporter-deficient Chinese hamster ovary cell mutant. J. Biochem. (Tokyo) 1998 724 171-178. 

Ishida N, Yoshioka S, lida M, Sudo K, Miura N, Aoki K, Kawakita M. Indispensability of transmembrane domains of Golgi UDP-Galactose transporter as revealed by analysis of genetic defects in UDP-Galactose transporter-deficient murine Had-I mutant cell lines and construction of deletion mutants. J. Biochem. (Tokyo) 1999 726 1107-1117. 

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