trans

TRANSPORT SYSTEMS 203
adenine. Like Plasmodium, T. gondii has on its
plasma membrane a V-type H -pump that
extrudes H from the cytosol into the PV, but
whether this gradient is used by TgAT to drive
transport is not yet known. At present the genes
or cDNAs for other nucleoside and nucleobase
transporters have not been identified within
the T. gondii genome, but with the advent of
a genome sequencing initiative it is likely that
other nucleoside transporter sequences will
emerge.
Eimeria, Cryptosporidia, and Babesia
The purine translocation pathways of these
parasites have received little attention. Like
Plasmodium, Babesia bovis induces a new permeation
pathway within the infected erythrocyte,
but very little is known about uptake into
the parasitophorous vacuole and translocation
across the parasite plasma membrane.
Virtually nothing is known about purine and
pyrimidine translocation into Eimeria and
Cryptosporidium. A recent screen of the relevant
parasite genome databases suggests that
both genera accommodate ENT homologs.
Kinetoplastida
Leishmania and Crithidia
Leishmania parasites have a digenetic lifecycle
in which the flagellated extracellular
promastigote persists in the midgut of the
sandfly and the aflagellate amastigote resides
within the acidic phagolysosome of the mammalian
host macrophage. Most of our knowledge
about the available transport of purines
and pyrimidines in Leishmania has been
derived from studies with the promastigote
form. Two distinct nucleoside transport loci
have been genetically defined in L. donovani.
These two transporters display non-overlapping
substrate specificities and recognize either
adenosine and pyrimidine nucleosides (LdNT1)
or inosine and guanosine (LdNT2) with high
affinity. These observations have also been
extrapolated to other Leishmania spp. and
Crithidia spp. Transfection of a L. donovani
cosmid library into nucleoside transportdeficient
L. donovani provided an avenue for
cloning the LdNT1 and LdNT2 genes by functional
rescue of the nucleoside transportdeficient
phenotype. The LdNT1 locus, which
consists of two nearly identical copies (LdNT1.1
and LdNT1.2), and LdNT2 encode ENT-type
proteins of 491 and 499 amino acids, respectively,
and display limited sequence identity to
each other and to the human ENTs. The functional
expression of these genes within nucleoside
transport-deficient L. donovani confirmed
that these proteins are mutually exclusive, highaffinity
transporters for adenosine–pyrimidine
nucleosides and inosine–guanosine, respectively.
Although LdNT1and LdNT2 share similarity
with hENT1 in their overall predicted
topological structures and signature sequences,
electrophysiological studies in Xenopus oocytes
indicate that they are not equilibrative but
rather function as proton symporters. The
homologous genes have now been cloned
from C. fasciculata.
It is clear from manipulation of the cultivation
medium with various exogenous purine
nucleobases and nucleosides that Leishmania
is capable of transporting and utilizing any preformed
purine or purine nucleoside, with the
exception of xanthosine. Adenine and hypoxanthine
transport has been demonstrated in
L. braziliensis panamensis, but these activities
were much reduced compared to the transport
of nucleosides. Since purine base transport in
Crithidia spp. and other Trypanosomatidae
is significantly enhanced under conditions of
purine depletion, it is entirely possible that at
BIOCHEMISTRY AND CELL BIOLOGY: PROTOZOA