Apoptosis-inducing factor-like (AIFL) protein contains a Rieske domain and pyridine nucleotide-disulfide oxidoreductase (Pyr_redox) domain that shows 35% homology to that of apoptosis-inducing factor (AlF) protein. We identified a novel major transcript of the medaka (Oryzias latipes) AIFL gene that retained intron 4 (AIFL-14) in embryos and tissues from adult fish. The product of this transcript, AIFL-I4 protein, lacked the Pyr_redox domain because of a nonsense codon in intron 4. Both AIFL-I4 and full-length AIFL (fAIFL) transcripts were highly expressed in the brain and late embryos, and relative fAIFL and AIFL-I4 expression levels differed among tissues. Transient expression of AIFL-I4 and fAIFL tagged with GFP showed that AIFL-I4 localized in the nucleus, while fAIFL localized throughout the cytoplasm. We also found that overexpression of AIFL-14 induced a change in mitochondrial morphology and suppression of cell proliferation. AIFL-I4 mutant with a lesion in [2Fe-2S] cluster binding site of the Rieske domain did not induce these phenotypes. This report is the first to demonstrate nuclear localization of a Rieske-type protein translated from the AIFL gene. Our data suggested that the [2Fe-2S] cluster binding site was essential for the nuclear localization and involved in mitochondrial morphology and suppression of cell proliferation. (C) 2011 Elsevier Inc. All rights reserved.