Highly divergent amino termini of the homologous human ALR and yeast scERV1 gene products define species specific differences in cellular localization

The yeast scERV1 gene product is involved in the biogenesis of mitochondria and is indispensable for viability and regulation of the cell cycle. Recen tly the general importance of this gene for the eukaryotic cell was shown b y the identification of a structural and functional human homologue, The ho mologous mammalian ALR (Augmenter of Liver Regeneration) genes from man, mo use and rat are involved in the phenomenon of liver regeneration. A low exp ression rate of the genes is found in all investigated cells and mammalian tissues but it is specifically induced after damage of liver organs and is especially high during spermatogenesis, The alignment of the different prot eins identifies a highly conserved carboxy terminus with more than 40 % ide ntical amino acids between yeast and mammals, The conserved carboxy terminu s is functionally interchangeable between distantly related species like ye ast and man. In contrast, the amino terminal parts of the proteins display a high degree of variability and significant differences even among closely related spec ies. This finding leads to the problem whether the amino termini have compa rable or divergent functions in different species. In this study we demonstrate by heterologous complementation experiments in yeast that the complete human ALR protein with its own amino terminus is n ot able to substitute for the yeast scERV1 protein. Fusion proteins of Alrp and scErv1p with the green fluorescence protein were created to investigat e the respective subcellular localizations of these homologous proteins in yeast and human cells. In yeast cells human Alrp accumulates in the cytopla sm in contrast to yeast scErv1p that is preferentially associated with yeas t mitochondria. Comparable studies with human cells clearly show that the h omologous human Alrp is located in the cytosol of these cells, Fractionatio n experiments and antibody tests with yeast and human mitochondria and cell ular extracts verify these findings.