trans

ANTIGENIC VARIATION AT THE STRUCTURAL AND FUNCTIONAL LEVEL 91
is a venereal disease of horses in Africa. The predominantly
tsetse-transmitted trypanosomes
can also be passed directly from mammal to
mammal, by syringe, by carnivorous eating
habits, and possibly transplacentally.
Trypanosoma brucei is the most widely studied
of the African trypanosomes, because it is
by far the most convenient laboratory model.
Although it is the least prevalent of the three
major species of veterinary importance (the
other two being T. congolense and T. vivax), it is
the species that contains the two recognized
subspecies that infect humans, T. brucei rhodesiense
and T. brucei gambiense. The entire life
cycle of T. brucei can be reproduced in the laboratory,
using rodents and tsetse, but not yet
entirely in cell culture. Both the bloodstream
forms and the tsetse midgut ‘procyclic’ form
can be readily cultivated in vitro, which allows
genetic manipulations to be performed in a relatively
facile manner. Relatively facile means
they can be genetically manipulated with about
10% the convenience of Saccharomyces cerevisiae,
but more than ten times the ease with
which similar manipulations can be performed
in the human malaria parasite Plasmodium
falciparum. Other apicomplexan parasites,
especially Toxoplasma gondii, can be more
readily manipulated than Plasmodium, and
can be informative about the functions of
some genes and organelles that are conserved
between Plasmodium and Toxoplasma.
ANTIGENIC VARIATION AT
THE STRUCTURAL AND
FUNCTIONAL LEVEL
The variant surface glycoprotein coat
Animal-infective bloodstream and tsetse
salivary-gland ‘metacyclic’ trypanosomes are
characterized by an electron-dense ‘surface
coat’ (Figure 5.1). The surface coat of an individual
trypanosome consists of about 10 million
molecules of a single molecular species of variant
surface glycoprotein (VSG). Antigenic variation
involves the sequential expression of coats
composed of different VSGs. Mice can be totally
protected against homologous challenge by
FIGURE 5.1 The surface structure of T. brucei bloodstream forms. A cross-section through the cell body and the
flagellum shows the cell membrane with its underlying microtubular cytoskeleton and overlying surface coat at
(A) low and (B) high magnifications. The approximate cross-sectional dimensions in (A) are 1 2 m.
MOLECULAR BIOLOGY