Multiple transcripts for the human cardiac form of the cGMP-inhibited cAMP phosphodiesterase

cDNAs for two distinct Type III cGMP-inhibited (cGI) cyclic nucleotide phosphodiesterases (PDE), designated cGIP1 and cGIP2, were previously cloned from rat adipose and human cardiac cDNA libraries, respectively. In this study, another cDNA (approximately 4.0 kilobase (kb)) encoding a cGI-PDE of 74 kDa (658 amino acids) was isolated from a human placental cDNA library. The nucleotide sequence of its open reading frame was virtually identical to a corresponding region in the 3' portion of the cardiac cGIP2 cDNA (approximately 7.6 kb) which encoded a approximately 125-kDa cGI-PDE (1141 amino acid). Northern blots and RNase protection assays revealed a prominent 4.4-kb transcript and a 7.6-kb transcript in human placenta. The transcription start site of the 4.4-kb transcript was assigned to cardiac cDNA nucleotide 1292, the putative beginning of exon 3 of the human cGIP2 gene, with a potential translation initiation site 183 bases downstream, as determined by RNase protection assay. The 5'-flanking region of the 4.4-kb transcript exhibited promoter activity in HeLa cells which expressed the 4.4-kb transcript, and contained a TATAA sequence 35 base pairs upstream from the tentative transcription start site. Recombinant cGI-PDEs, expressed in Sf9 cells from the 7.6- and 4.0-kb cDNA, exhibited differences in their subcellular localization and Km for cGMP. Thus, in human tissues, alternative transcription may contribute to generating at least two cGIP2 isoforms, cytosolic and membrane-associated cGI-PDEs with different Km values for cGMP.