In our recent study using the HP-TRFAMsystem,we showed that the
S. violacea membrane was extremely rigid (S = 0.9) and the rotational
acyl chain motion was highly restricted (Dw = 5.4 μs−1) even at
0.1 MPa and 10 °C (Fig. 4) [22,26]. Importantly, S. violacea membrane
is imperturbable over a wide range of hydrostatic pressures with respect
to small effects on S and Dw. This is in sharp contrast with E. coli
membrane, of which S and Dw are markedly influenced by increasing
pressure (Fig. 3). The loss of EPA mandated a compensatory increase
in palmitoleic acid (C16:1) by two-fold in the S. violacea cellmembrane
[22]. Analogous to the POPC and E. coli cell membrane, S. violacea cell
membrane lacking EPA showed a lower S and a higher Dw compared
with the membrane containing EPA, possibly occupying more spaces
within the lipid bilayer (Fig. 4). Consequently, the EPA-less membrane
undergoes considerable packing with increasing pressure. In this situation,
membrane-embedded proteins in the EPA-less membrane could
be more perturbed by high pressure due to the changes in membrane
structure. In this analogy, high-pressure sensitivity of E. coli could be
in part explained by enhanced packing of the membrane and reduced
acyl-chain motion under high pressure that could result in impairment
in membrane protein functions. Some microorganisms would increase
membrane fluidity to survive in cold, high-pressure environments. In
our scheme, however, maintaining membrane physical properties at a
certain level, even highly rigid, under a wide range of pressures could
be viewed as another way of adaptation to high-pressure environments
possibly allowing the deep-sea piezophiles to stand against pressure fluctuations
in vertical currents in deep sea. This is in accordance with the
above-mentioned observation in L. plantarum that the HorA multidrug
resistant transporter is more resistant to high pressure when the membrane
is in the gel phase [33]. High pressure is assumed to weaken
protein–lipid interactions, induce oligomer dissociation, and, in some
cases, release membrane proteins. There are a number of excellent
reviews that may be consulted for greater perspective [2–4]. Currently,
not many membrane-embedded proteins have been investigated, with
respect to the role of EPA in their functions, but it can be readily understood
that high pressure differentially influences membrane proteins by changing the dynamic membrane structures and phase behavior.
Although many issues remain to be elucidated, the approaches by
means of HP-TRFAM may offer clues linking the structural aspects of
membranes and membrane protein functions in a quantitative manner.