Compressible compartments

Water, solutes and most soluble macromolecules are of low compressibility under hydrostatic pressures below 1000 atm. Compression of biological membranes at this range originates predominantly from intermolecular free space, which is characteristic of lipid and protein assemblies. The lipid layer in biological membranes is readily condensed by pressure. As a result, the free space between the phospholipid molecules is reduced and the lipid structure becomes more ordered and viscous with respect to the magnitude of the applied pressure. In the extreme case, the lipid can undergo a phase transition to an ordered gel phase, which is highly dense and therefore of low compressibility. Upon decompression, the lipid layer returns practically to its initial state.

Protein assemblies and multimers also include appreciable free space which can be released under pressure. In this case, the overt effect of pressure is not gradual, as in the case of lipids, but is quite abrupt in correspondence with the pressure threshold which induces dissociation to monomers. Reduction of pressure below this threshold level will then allow the proteins to reassemble.

When hydrostatic pressure in the range of 1000— 1500 atm is imposed on cells, the increase in order and microviscositv of the membrane lipids is similar in magnitude to that induced by doubling the level of membrane cholesterol. The cytoskeletal network, which is composed of microtubules and microfilaments, is almost completely dissociated to form soluble monomers. As a result, some of the membrane proteins are displaced vertically, and a part of them is shed off into the aqueous domain, due to excessive displacement and dissociation from the cytoskeletal anchorage. Most other membrane proteins aggregate nonspecifically to form segregated domains (Figure 1). When a slow decompression is allowed, the affected cellular systems readjust with respect to the ambient pressure through a series of equilibria until atmospheric pressure is attained. Along with decompression, the protein aggregates will dissociate in part, while proteins of some affinity will remain associated as the cytoskeletal boundaries arc reassembled.

Another, somewhat unexpected, effect of hydrostatic pressure is on protein synthesis. Similarly to protein assemblies, complexes between various suppressors and genetic matrices can dissociate under pressure and immediately evoke protein synthesis. This process can be classified as stress induced (e.g. analogously to heat shock protein (hsp) synthesis). Some of the pressure-induced proteins presumably take part in the surface presentation of antigens, as shown in Figure 2, and thus contribute to the apparent immunogenicity. The modulation in surface antigens which follow such a process, in particular the overt augmentation of major histocompatibility complex (MHC) presentation, is a combination of lateral and vertical displacements, presented in Figure 1,

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