consistent with increasing the capacity for
greater glucose passage to the fetus, during
the period when fetal energy demands are
greatest.
ASCT family of zwitterionic amino acid
transportersAs noted above, Na+-dependent system ASC
transports alanine, serine, cysteine and
other neutral -amino acids. ASCT1 cDNA,
originally cloned from human brain tissue,
encodes system ASC-like activity and shares
nearly 40% amino acid sequence identity
with the anionic EAAT transporters. A sec-
ond cDNA (ASCT2) has been cloned from
mouse and human tissues that encodes sys-
tem ASC-like activity. Human ASCT2 shares
61% identity with human ASCT1 and dis-
plays a broader pattern of substrate recogni-
tion than, but similar transport function to,
ASCT1. The ASCT1 and ASCT2 cDNAs
encode open reading frames of 532 and 541
amino acids, respectively. As with the
EAAT family, hydrophobicity modelling of
the ASCT transporter sequences predict
six well-defined transmembrane sequences
near the N-terminus of the protein and
additional, less well-defined, hydrophobic
stretches near the C-terminus. Although sys-
tem ASC normally recognizes neutral amino
acids, at pH ≤5.5, only glutamate and aspar-
tate are substrates, not neutral amino acids.
This pH-dependent recognition pattern is
displayed by ASCT1 and ASCT2. The trans-
port of anionic amino acids by ASCT1 and
ASCT2 at low pH appears to be an example
of similarities in structure/function that
exist between the EAAT and ASCT families.
System ASC activity has been
identified in nearly every mammalian
tissue tested, and can account for the
majority of uptake of several neutral amino
acids in a number of cell types. For many
years, it was thought that Na+-dependent
system ASC functioned as a concentrative
transporter. However, based on functional
expression studies of ASCT1 in defollicul-
ated Xenopus laevis oocytes, it is now
thought that ASCT1-mediated system ASC
activity is that of an obligatory exchanger
system (Zerangue and Kavanaugh, 1996).
As such, one extracellular amino acid
would be exchanged for one intracellular
transporter, resulting in no net accumula-
tion of amino acids. Theoretically, how-
ever, because of potential differences in
extracellular and intracellular binding
affinities, the concentration of a given
amino acid in the cytosol could be
achieved at the expense of others being
transported out of the cell. If system ASC is
confirmed to be an obligatory exchanger in
mammalian cell expression models, then
system A, which has not been cloned,
would remain as the only identified Na+-
dependent transporter capable of concen-
trative neutral amino acid uptake in
non-polarized cells.
ASCT1 mRNA is expressed highly in
the brain, skeletal muscle and pancreas,
moderately in the heart, and very weakly
in liver, lung, placenta and kidney human
tissues. The expression of ASCT2 mRNA
appears limited to cells of the lung,
skeletal muscle, kidney adipose, pancreas,
placenta, testes and large intestine. The
fact that ASCT1 and ASCT2 have not been
reported in the small intestine, but that
system ASC activity is high in intestinal
tissue, suggests that other members of this
family of neutral amino acid transporters
have yet to be identified.NBAT/4F2hc family of zwitterionic and
cationic amino acid transportersA new class of transport-related proteins
has been identified in several animal
species after induction of system bo,+(Na+-
independent uptake of both zwitterionic (o)
and cationic (+) amino acids) in oocytes
following expression of size-fractionated
kidney mRNA (Deves and Boyd, 1998).
This chapter will refer to all cDNAs that
encode system bo,+activity as NBAT (neu-
tral basic amino acid transport), even
though cDNAs from several species were
cloned and named independently (Table
1.3). NBAT is predicted to have 1–4 mem-
brane-spanning domains, thereby differing
from most solute transporters, which areAmino Acid and Peptide Transport Systems 13