trans

92 ANTIGENIC VARIATION
immunization with individual purified VSGs.
Purification turned out to be far easier than
anticipated, for two reasons. Firstly, the derivation
of trypanosome clones that switch infrequently
allows large quantities of individual
VSGs to be purified from trypanosome populations
that are 99.9% homogeneous. Secondly,
although the VSG is firmly attached to the surface
membrane of intact trypanosomes, it is rapidly
released when the membrane is breached.
For many years this convenient behavior was a
paradox. Amino acid sequencing of selected
regions of several VSGs led to the prediction
that the carboxy terminus of purified VSG had
arisen by cleavage of a larger precursor. This
prediction was confirmed when the corresponding
genes were cloned and sequenced. The
finding that the purified protein terminated in
an amide-(peptide) bonded residue of ethanolamine
confirmed that proteolytic cleavage had
not occurred during VSG purification and
led to the identification of a covalently linked
lipid, whose complete structure was ultimately
determined, providing the prototype for a
novel mode of protein–membrane attachment,
the glycosylphosphatidylinositol (GPI) anchor
(Figure 5.2). GPI anchors have now been
found on proteins having a wide range of functions,
in all eukaryotic cells, but they appear to
be particularly abundant in parasitic protozoa,
for reasons that have not been resolved. A consensus
is emerging that GPI anchoring may
be important for locating certain proteins to
specialized subdomains of the plasma membrane.
Certain aspects of GPI anchor structure
are conserved in all eukaryotic cells, from
FIGURE 5.2 (See also Color Plate 2) (Left) Structure of the minimal (‘core’) VSG GPI anchor and (right) threedimensional
structures of the amino-terminal domains of VSG MITat 1.2 (left) and ILTat 1.24 (right). The dark and
light ribbons indicate the atomic traces of the two identical units comprising the dimeric domain. The amino
termini are at the top of the molecule, which forms the outer face of the coat. The bottom of this domain is linked
to the carboxy-terminal domain, for which a structure is not available, which is then linked to the GPI anchor. The
image was kindly provided by Ms Lore Leighton.
MOLECULAR BIOLOGY