Molecular adaptation and salt stress response of Halobacterium salinarum cells revealed by neutron spectroscopy.

Halobacterium salinarum is an extreme halophile archaeon with an absolute requirement for a multimolar salt environment. It accumulates molar concentrations of KCl in the cytosol to counterbalance the external osmotic pressure imposed by the molar NaCl. As a consequence, cytosolic proteins are permanently exposed to low water activity and highly ionic conditions. In non-adapted systems, such conditions would promote protein aggregation, precipitation, and denaturation. In contrast, in vitro studies showed that proteins from extreme halophilic cells are themselves obligate halophiles. In this paper, adaptation via dynamics to low-salt stress in H. salinarum cells was measured by neutron scattering experiments coupled with microbiological characterization. The molecular dynamic properties of a proteome represent a good indicator for environmental adaptation and the neutron/microbiology approach has been shown to be well tailored to characterize these modifications. In their natural setting, halophilic organisms often have to face important variations in environmental salt concentration. The results showed deleterious effects already occur in the H. salinarum proteome, even when the external salt concentration is still relatively high, suggesting the onset of survival mechanisms quite early when the environmental salt concentration decreases.

Neutron scattering: a tool to detect in vivo thermal stress effects at the molecular dynamics level in micro-organisms.

In vivo molecular dynamics in Halobacterium salinarum cells under stress conditions was measured by neutron scattering experiments coupled with microbiological characterization. Molecular dynamics alterations were detected with respect to unstressed cells, reflecting a softening of protein structures consistent with denaturation. The experiments indicated that the neutron scattering method provides a promising tool to study molecular dynamics modifications in the proteome of living cells induced by factors altering protein folds.

Neutron scattering reveals extremely slow cell water in a Dead Sea organism.

Intracellular water dynamics in Haloarcula marismortui, an extremely halophilic organism originally isolated from the Dead Sea, was studied by neutron scattering. The water in centrifuged cell pellets was examined by means of two spectrometers, IN6 and IN16, sensitive to motions with time scales of 10 ps and 1 ns, respectively. From IN6 data, a translational diffusion constant of 1.3 x 10(-5) cm(2) s(-1) was determined at 285 K. This value is close to that found previously for other cells and close to that for bulk water, as well as that of the water in the 3.5 M NaCl solution bathing the cells. A very slow water component was discovered from the IN16 data. At 285 K the water-protons of this component displays a residence time of 411 ps (compared with a few ps in bulk water). At 300 K, the residence time dropped to 243 ps and was associated with a translational diffusion of 9.3 x 10(-8) cm(2) s(-1), or 250 times lower than that of bulk water. This slow water accounts for approximately 76% of cell water in H. marismortui. No such water was found in Escherichia coli measured on BSS, a neutron spectrometer with properties similar to those of IN16. It is hypothesized that the slow mobility of a large part of H. marismortui cell water indicates a specific water structure responsible for the large amounts of K(+) bound within these extremophile cells.