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Activity regulation of the betaine transporter BetP of Corynebacterium glutamicum in response to osmotic compensation

Institute of Biochemistry, University of Köln, Zülpicher Str. 47, 50674 Köln, Germany.
Biochimica et Biophysica Acta (Impact Factor: 4.66). 01/2005; 1667(2):229-40. DOI: 10.1016/j.bbamem.2004.10.012
Source: PubMed

ABSTRACT As a response to hyperosmotic stress bacterial cells accumulate compatible solutes by synthesis or by uptake. Beside the instant activation of uptake systems after an osmotic upshift, transport systems show also a second, equally important type of regulation. In order to adapt the pool size of compatible solutes in the cytoplasm to the actual extent of osmotic stress, cells down-regulate solute uptake when the initial osmotic stress is compensated. Here we describe the role of the betaine transporter BetP, the major uptake carrier for compatible solutes in Corynebacterium glutamicum, in this adaptation process. For this purpose, betP was expressed in cells (C. glutamicum and Escherichia coli), which lack all known uptake systems for compatible solutes. Betaine uptake mediated by BetP as well as by a truncated form of BetP, which is deregulated in its response to hyperosmotic stress, was dissected into the individual substrate fluxes of unidirectional uptake, unidirectional efflux and net uptake. We determined a strong decrease of unidirectional betaine uptake by BetP in the adaptation phase. The observed decrease in net uptake was thus mainly due to a decrease of Vmax of BetP and not a consequence of the presence of separate efflux system(s). These results indicate that adaptation of BetP to osmotic compensation is different from activation by osmotic stress and also different from previously described adaptation mechanisms in other organisms. Cytoplasmic K+, which was shown to be responsible for activation of BetP upon osmotic stress, as well as a number of other factors was ruled out as triggers for the adaptation process. Our results thus indicate the presence of a second type of signal input in the adaptive regulation of osmoregulated carrier proteins.

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    • "We have previously studied in detail the function of compatible solute uptake in the instant response upon hyperosmotic shock, on the one hand, and also the significance of BetP, the major osmoresponsive uptake system in C. glutamicum, to the osmotic adaptation under continuous presence of hyperosmotic stress [13]. As a soil bacterium, C. glutamicum is frequently exposed to osmotic stress; furthermore its significance as a model organism in biotechnology renders it an interesting object to study osmoregulation. "
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    ABSTRACT: Based on sequence similarity, the mscCG gene product of Corynebacterium glutamicum belongs to the family of MscS-type mechanosensitive channels. In order to investigate the physiological significance of MscCG in response to osmotic shifts in detail, we studied its properties using both patch-clamp techniques and betaine efflux kinetics. After heterologous expression in an Escherichiacoli strain devoid of mechanosensitive channels, in patch-clamp analysis of giant E. coli spheroplasts MscCG showed the typical pressure dependent gating behavior of a stretch-activated channel with a current/voltage dependence indicating a strongly rectifying behavior. Apart from that, MscCG is characterized by significant functional differences with respect to conductance, ion selectivity and desensitation behavior as compared to MscS from E. coli. Deletion and complementation studies in C. glutamicum showed a significant contribution of MscCG to betaine efflux in response to hypoosmotic conditions. A detailed analysis of concomitant betaine uptake (by the betaine transporter BetP) and efflux (by MscCG) under hyperosmotic conditions indicates that MscCG may act in osmoregulation in C. glutamicum by fine-tuning the steady state concentration of compatible solutes in the cytoplasm which are accumulated in response to hyperosmotic stress.
    Biochimica et Biophysica Acta 11/2010; 1798(11):2141-9. DOI:10.1016/j.bbamem.2010.06.022 · 4.66 Impact Factor
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    • "We have previously studied in detail the function of compatible solute uptake in the instant response upon hyperosmotic shock, on the one hand, and also the significance of BetP, the major osmoresponsive uptake system in C. glutamicum, to the osmotic adaptation under continuous presence of hyperosmotic stress [13]. As a soil bacterium, C. glutamicum is frequently exposed to osmotic stress; furthermore its significance as a model organism in biotechnology renders it an interesting object to study osmoregulation. "
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