AMY-1 has been identified by us as a c-Myc-binding protein and was found to stimulate c-Myc transcription activity. AMY-1 was also found to be associated with A-kinase anchor protein 84/149 (S-AKAP84/AKAP149) in the mitochondria in somatic cells and sperm, suggesting that it plays a role in spermatogenesis. To elucidate the functions of AMY-1, a cDNA encoding a novel protein named AAT-1 was obtained. Three isoforms of AAT-1, AAT-1a, -b and -g were found to be derived from an alternative

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... of the transcripts of the aat-1 gene, which was mapped at human chromosome 3q13-3q21. AAT-1 is specifically expressed in the testis during the course of spermatogenesis and is also present in the spermatid and mature sperm, as is AMY-1. 1-3) AAT-1a binds to and is colocalized in mitochondria with AMY-1 in human HeLa and mouse GC-1 cells. 4) Furthermore, AAT-1a binds to the N-terminal half of S-AKAP84/AKAP149 in a complex with AMY-1 and a regulatory subunit (RII) of cAMP-dependent kinase (PKA), in which AAT-1a is associated with RII via S-AKAP84/ AKAP149, in the rat testis and HeLa cells. AAT-1a was found to stimulate a phosphorylation activity of PKA and AAT-1 itself was phosphorylated by PKA in vivo and in vitro. 4) These results suggest that both AAT-1 and AMY-1 play roles in spermatogenesis. In this study, to further characterize structure and functions of AAT-1, other splicing isoforms of AAT-1 were identified. Binding activities of AAT-1 isoforms to AMY-1 and their cellular localizations were analyzed. A novel protein, AAT-1, was identified as a AMY-1-binding protein and three splicing variants of AAT-1, AAT-1a a, -b b and -g g were identified. The function of AAT-1 is thought to be related to spermatogenesis. In this study, we further identified other splicing isoforms of AAT-1, AAT-1L, AAT-1M and AAT-1S, consisting of 767, 603 and 252 amino acids, respectively. These isoforms were found to use a promoter different from that used by AAT-1a a, -b b and -g g in the aat-1 gene, which contains 20 exons. Only 60 amino acids in the C-terminal portion of AAT-1 derived from exons 15-17 are common among AAT-1L, AAT-1M, AAT-1S and AAT-1a a. While AAT-1a a is specifically expressed in the testis, AAT-1L, AAT-1M, AAT-1S were found to be differentially expressed in human tissues. All of the isoforms of AAT-1 were found to bind to and colocalized with AMY-1 in human cells. While AAT-1L and AAT-1M were found to be localized diffusely in the cytoplasm, AAT-1S, like AAT-1a a, was found to be localized in the mitochondria-like structure, suggesting different roles of AAT-1 isoforms in cells.