Molecular cloning and expression of a mouse muscle cDNA encoding adenylosuccinate synthetase

Adenylosuccinate synthetase (EC 6.3.4.4) catalyzes the first step in formation of AMP from IMP. At least two isozymes exist in vertebrate tissue. An acidic form, present in most tissues, has been suggested to be involved in de novo biosynthesis while a basic isozyme, which predominates in muscle, appears to function in the purine nucleotide cycle. Antibodies specific for the basic isozyme detect a single protein in mouse tissues with highest levels in skeletal muscle, tongue, esophagus, and heart tissue consistent with a role for the enzyme in muscle metabolism. A series of degenerate oligonucleotides were constructed based on peptide sequences from purified rat muscle enzyme and then used to clone a mouse muscle cDNA encoding the basic isozyme. The clone contains a open reading frame of 1356 bases with 452 amino acids. Northern analysis of RNA from mouse tissues showed a tissue distribution similar to that of the protein, indicating a high level of gene expression in muscle. Transfection of COS cells with the mouse muscle cDNA allows expression of a functional protein with a molecular mass of approximately 50 kDa, consistent with the open reading frame and the size of the isolated rat enzyme. The deduced amino acid sequence of the mouse synthetase is 47 and 37% identical to the synthetase sequences from Dictyostelium discoideum and Escherichia coli, respectively. The availability of antibodies and cDNA clones specific for the basic isozyme of adenylosuccinate synthetase from muscle will facilitate future genetic and biochemical analysis of this protein and its role in muscle physiology.