NAD-DEPENDENT METHYLENETETRAHYDROFOLATE DEHYDROGENASE-METHENYLTETRAHYDROFOLATE CYCLOHYDROLASE IS TARGETED TO THE CYTOPLASM IN INSECT-CELL LINES

Cytosolic NADP-dependent methylenetetrahydrofolate dehydrogenase-cyclo hydrolase synthetase and the mitochondrial NAD-dependent methylenetetr ahydrofolate dehydrogenase-cyclohydrolase (NMDMC) are differentially e xpressed during insect development although both enzymes are detectabl e at all stages. In contrast, cell lines derived from a variety of ins ect species express high levels of NMDMC but undetectable levels of th e NADP-dependent enzyme. Northern analysis indicates the NMDMC message is expressed at levels 50-100 times higher in a Drosophila cell line compared to adult flies. RNase protection showed the predominance of s hortened transcripts that require initiation at a downstream AUG produ cing a truncated protein that lacks a mitochondrial targeting sequence , These changes in expression effectively exchange the cytosolic NADP- dependent dehydrogenase for one with NAD specificity.