Biology Reference
In-Depth Information
membranes consisting of various raft (e.g. SM/cholesterol/ganglioside) and non-raft (e.g.
various unsaturated phospholipids). With fluorescent cholora toxin, raft domains light up.
If the same model bilayer membranes are imaged by AFM, overlapping domains are
observed. Both techniques identify the same membrane patches as being raft domains.
Many other examples simultaneously imaging rafts by fluorescence and AFM can be found
in the literature (for example see
[47]
).
D. HOMEOVISCOUS ADAPTATION
Many of the essential membrane properties discussed inChapters 9, 10, and 11must bemain-
tained within narrow constraints for life to continue. The well-studied examples of disease and
aging are often associated with deleterious alterations of membrane physical properties. For
warm-blooded (homeothermic) animals,membranes are naturallykept between a narrow, func-
tional temperature range and are not exposed to harmful temperature fluctuations. However,
most organisms on planet Earth have no internal temperature control mechanism (they are
cold-blooded or poikilothermic) and are at the whim of the external environment. Examples
of poikilothermic organisms are reptiles, fish, plants, fungi, bacteria, protists, and even to
some extent hibernating mammals. Diving mammals face similar problems, as an increase in
environmental pressure affects membranes in a similar way to a decrease in temperature. All
organisms must be able to control environmental insults to assure proper membrane function.
The effort to maintain proper membrane properties is known as homeoviscous adaptation (HVA),
a general term where all temperature-dependent properties are lumped into 'membrane
viscosity'. Viscosity is roughly the inverse of 'fluidity' (see Chapter 9).
The concept of HVAwas first proposed in 1974 by Sinensky frommembrane lipid studies on
Escherichia coli
[48]
. HVA caught on quickly and was supported by many studies on arctic fish,
plants, fungi, and bacteria. Of particular importance was a series of papers by A.R. Cossins
[49,50]
. Cossins assessed membrane order (a measure of 'fluidity') for a series of organisms
of different body or habitat temperatures. The measured order parameters were found to
directly follow the organism's temperature [Antarctic fish (
1
C)
perch (15
C)
<
<
convict
cichlid (28
C)
pigeon (42
C)]
[51]
. These results indicated that 'evolutionary
adaptation to cold environments produces membranes of significantly lower order'
[51]
.
And this is at the heart of HVA theory.
In an excellent and thoughtful review, Jeffrey Hazel
[51]
agrees that lipids are indeed
involved in HVA, but suggests their effect is far more complex than standard HVA theory
would suggest. An abbreviated list of membrane properties that are hard to incorporate
into conventional HVA theory include: membrane remodeling; microdomain heterogeneity;
specificity of lipid
e
protein interactions and; proliferation of mitochondrial and sarcoplasmic
reticular membranes. Several membrane properties, closely linked to fluidity and likely
involved in HVA, are briefly discussed below.
rat (37
C)
<
<
Acyl Chain Length
Shorter chain lengths have lower T
m
s than longer chain lengths (Chapter 4) and so accu-
mulate in membranes of organisms exposed to lower temperatures.
