A human homologue of the yeast GST1 gene codes for a GTP-binding protein and is expressed in a proliferation-dependent manner in mammalian cells

A human homologue (GST1-Hs) of the yeast GST1 gene that encodes a new GTP-binding protein essential for the G1-to-S phase transition of the cell cycle was cloned from the cDNA library of human KB cells. The GST1-Hs cDNA contained a 1497 bp open reading frame coding for a 499 amino acid protein with mol. wt 55,754 and with the amino acid sequence homologies of 52.3 and 37.8% to the GST1 protein and polypeptide chain elongation factor EF1 alpha respectively. The regions potentially responsible for GTP binding and GTP hydrolysis were conserved in the GST1-Hs protein as well. When expressed in yeast cell, the GST1-Hs gene could complement the ts phenotype of yeast gst1 mutant. GST1-Hs and its mouse homologue were expressed in human fibroblasts and in various mouse cell types respectively, at relatively low levels in their quiescent states, and the level of those expressions increased rapidly, prior to the onset of DNA replication and the total RNA synthesis, when human or mouse fibroblasts were progressed out of the growth-arrested state by the addition of serum. A possible role of GST1-Hs in mammalian cell growth is discussed.