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protein. More frequently, a membrane may well be dominated by one
species of an integral membrane protein but with additional proteins that
are more loosely attached. Those can be removed by centrifugation fol-
lowing detergent treatment and conditions can be applied which stimulate
contacts leading to periodic packing. An example is the purified Na + , K + -
pump, which forms 2D arrays in its native membrane after incubation
with vanadate (Hebert et al ., 2001).
Soluble proteins
The basic principle behind making 2D crystals of soluble proteins is to
concentrate them to the fluid-air interface like an oil film spreading on a
water surface of a compartment (Uzgiris, Kornberg, 1983). The protein
should have an affinity for the surface. A phospholipid monolayer may act
as the scaffold for the protein. In some cases, like for bacterial toxins, the
protein may have an intrinsic affinity for the lipid. Insertion of the protein
can be measured in a Langmuir trough as an increase in lateral pressure.
For some proteins lipid interaction is dependent on concentration of ions,
e.g. annexins have increased lipid affinity in the presence of Ca 2+ (Brisson
et al ., 1991). Attaching a special chemical group to the lipid head groups
could be used to induce specific interaction with proteins having affinity
for that particular chemical group. A general approach along this line for
proteins containing a histidine tag has been to synthesize Ni-chelated
phospholipids (Kubalek et al ., 1994).
The 2D crystallization trials can be performed in small containers
holding a few milliliters of solution. After spreading the phospholipid at
the buffer-air interface, the protein is injected into the subphase. The con-
tainer should be kept in a humid environment.
Properties of 2-D crystals
Symmetry
Two-dimensional crystals are objects which are periodic in a plane with
an extension of one or a few molecular layers along the normal to this
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