Molecular Cloning and characterization of placental tissue protein 18 (PP18a)/human mitochondrial branched-chain aminotransferase (BCATm) and its novel alternatively spliced PP18b variant
Ng. Than et al., Molecular Cloning and characterization of placental tissue protein 18 (PP18a)/human mitochondrial branched-chain aminotransferase (BCATm) and its novel alternatively spliced PP18b variant, PLACENTA, 22(2-3), 2001, pp. 235-243
Five different insert-length cDNAs encoding for soluble placental tissue pr
otein 18 (PP18) variants were isolated by screening a human placental cDNA
library using monospecific anti-PP18 serum. Sequence analysis of the longes
t clone showed that the insert contains an open reading frame encoding for
a 392 residue-long protein with a 27 amino acid mitochondrial targeting seq
uence. The mature protein-designated PP18a-is 41.264 kDa consisting of 365
residues and is identical to the previously isolated and characterized PP18
antigen described in 1985. We also found a new, alternatively spliced cDNA
encoding for a 300 residue-long, 33.776 kDa protein, which was designated
PP18b. Alignment search of the protein databank showed that PP18a is almost
entirely identical to the human mitochondrial branched-chain aminotransfer
ase, while PP18b is its newly discovered splicing variant. We detected the
two PP18 variants in normal adult and fetal human tissues besides the mitoc
hondrial (only PP18a) and cytosolic (only PP18b) fractions of term placenta
with chemiluminescence Western blot analysis. The 41 kDa PP18a variant was
expressed ubiquitously, while the 33 kDa PP18b variant was found in smalle
r amounts in nearly all tissues. Trace amounts of the variants were present
in the sera of non-pregnant healthy controls, as well as in pregnant women
, but there was no real change in serum levels during pregnancy. In conclus
ion, PP 18 variants are not specific for the placenta. Aminotransferase act
ivity of placental origin PP18 antigens was verified by structural analysis
and by a coupled branched-chain aminotransferase/glutamate dehydrogenase a
ssay. (C) 2001 Harcourt Publishers Ltd.