Mitochondrial aspartate aminotransferase: direction of a single protein with two distinct functions to two subcellular sites does not require alternative splicing of the mRNA.

During differentiation of mouse 3T3-L1 fibroblasts to an adipocyte phenotype, the mitochondrial isoform of aspartate aminotransferase accumulates on the plasma membrane. The determination of whether this reflects translation of an alternatively spliced message lacking the mitochondrial leader sequence required cloning of the enzyme's uncommon a allele, for which these cells are homozygous. The 1.4-kb cDNA sequence of the a allele was obtained from oligo-dT-primed reverse-transcriptase PCR products amplified from FVB mouse RNA. It differed from the b allele at only 2 bp and one amino acid. By contrast, gene-specific primers generated an additional 1.4-kb fragment that differed from the b allele by approximately 1% of nucleotides, encoding four amino acid substitutions. This sequence proved to represent a recently diverged processed pseudogene. The presence of such pseudogenes can complicate interpretation of expressed-sequence-tag data and single-nucleotide-polymorphism genotyping studies. Using probes derived from the a allele, RNase protection analyses indicated that only a single message for the enzyme was present in 3T3-L1 fibroblasts and adipocytes, despite differences in subcellular protein distribution.