Biology Reference
In-Depth Information
(50
e
100 nm) invaginated membrane structures that superficially resemble coated pits. In
fact, in 1955 Yamada
[33]
proposed the descriptive name 'caveolae' which is Latin for little
caves. Caveolae were first described by the electron microscopist George Palade in 1953
and are abundant in many vertebrate cell types, especially endothelial cells and adipocytes
where they may account for between 30 to 70% of the total plasma membrane surface
area. Caveolae, however, are not a universal feature of all cells as they are totally absent in
neurons. Like lipid rafts, caveolae are partially characterized by being enriched in sphingo-
lipids and cholesterol and also participate in signal transduction processes
[34,35]
. In fact
caveolae are often described as being 'invaginated lipid rafts' that differ primarily by the
presence of a family of marker proteins called caveolins. But, similar to coated pits, caveolae
may even play a role in endocytosis.
2. Lipid Microdomains
Although macrodomains are large, stable, and relatively easy to image, most of a biological
membrane likely consists of many unstable heterogeneous patches of lipids and proteins
known as lipid microdomains. These structures are well known in model membrane systems
where they are driven by lipid lateral phase separations. However, if lipid microdomains
exist at all in biological membranes, they are fleeting and very small. Their possible existence
is based primarily on homologies with many experiments performed for decades on model
lipid monolayers and bilayers. The documented phase separations have been primarily
induced by changes in temperature, pressure, ionic strength, divalent cations, and cationic
peripheral proteins. In contrast to macrodomains, lipid microdomains have to date been
impossible to isolate and directly study as pure entities. Instead their existence is often
inferred from biophysical techniques where the experimental measurements demonstrate
co-existing multiple membrane populations, i.e. lipid microdomains.
In early experiments with sea urchin and mouse eggs (discussed in Chapter 9), Michael
Edidin employed lateral diffusion measurements using fluorescence recovery after photo-
bleaching (FRAP) to support the concept of membrane heterogeneity in membranes
[36]
.
Edidin's experiment followed the diffusion of two carbocyanine dyes, one with two short,
saturated lipid chains (C
10
,C
10
DiI) and one with two long saturated lipid chains (C
22
,C
22
DiI). The measured difference in diffusion rates between the two dyes was consistent with
the existence of lipid microdomains.
Membrane microdomains have also been inferred from the activity of reconstituted
enzymes. In one interesting model
[37,38]
, the sarcoplasmic reticulum Ca
2
þ
ATPase was
reconstituted into liposomes made from either zwitterionic DOPC (18:1, 18:1 PC), anionic
DOPA (18:1, 18:1 PA) or a DOPC/DOPA (1:1) mixture, and the enzyme activity measured.
Activity was high in DOPC and low in DOPA. For the DOPC/DOPA (1:1) liposome, an inter-
mediate activity was observed. Upon the addition of Mg
2
þ
to the Ca
2
þ
ATPase reconstituted
in DOPC, the activity did not change and remained high. In contrast, Mg
2
þ
reduced activity
of the Ca
2
þ
ATPase reconstituted into DOPA to zero. The precipitous decrease in activity was
attributed to Mg
2
þ
binding to the anionic DOPA and inducing an isothermal phase transition
resulting in the DOPA liposome being driven into the totally inactive gel state. When Mg
2
þ
was added to the DOPC/DOPA mixed liposome, the Ca
2
þ
ATPase activity was observed to
increase as DOPAwas removed from the fluid mixture as a gel. The induced isothermal phase
