Primate triosephosphate isomerase isozymes and gene structure.

Abstract

Triosephosphate isomerase (TPI, EC 5.3.1.1) a homo-dimeric enzyme that catalyzes the interconversion of dihydroxyacetone phosphate and glyceraldehyde 3-phosphate, is highly conserved through evolution. In hominoids (human and great apes), in addition to a constitutively expressed isozyme, a unique subunit (TPI-2) is expressed following mitogen stimulation of quiescent cells. Both isozymes are products of a single locus in humans, however there is no precursor-product relationship between TPI-1 and TPI-2. TPI-2 is distinguished from TPI-1 by its thermolability and its covalently altered electrophoretic mobility. Both subunits are the same molecular weight. TPI-2 is expressed within 6-8 hours following mitogen stimulation of peripheral human lymphocytes. TPI mRNA levels are increased several fold in these stimulated cells, however only one size mRNA is detectable by northern analysis of RNA from either resting or stimulated cells. The signals necessary for TPI-2 expression in mitogen stimulated cells are in the steps following the site of dexamethasone inhibition of cellular proliferation. TPI-2 expression is dependent on RNA synthesis but not on DNA synthesis. Neither TPI subunit was phosphorylated thus differential phosphorylation is not a mechanism for TPI-2 expression. Only a single TPI mRNA species was observed by S1 nuclease analysis of RNA from quiescent and proliferating cells. Thus, there is only one mRNA for both TPI-1 and TPI-2 or the difference(s) between the two mRNAs is too small to be detected by S1 nuclease analysis. The coding sequence of the TPI gene from rhesus macaque(Macacca mulatta) (a non TPI-2 expressing species) differs from the human TPI gene coding sequence (a TPI-2 expressing species) by 21 nucleotides, resulting in only two amino acids differences. Although one of these amino acid differences (human serine to rhesus asparagine) is a potential site for modification in the protein from humans and not rhesus, the presence of a serine residue at this position is not restricted to the TPI-2 expressing species, indicating that this residue is not related to the mechanism of TPI-2 synthesis. Although the mechanism for generation of TPI-2 remains elusive, these results provide further insight into the regulation of expression of this subunit.