We isolated cDNA for a novel protein IABP4 that binds specifically to a particular importin a protein whose expression is down-regulated by light in rice plants. IABP4 consists of a long N-terminal tetratricopeptide repeat (TPR) domain, the presence of which implies activity to form a protein complex, and a short C-terminal domain that contains three putative nuclear localization signals (NLSs). The deduced amino acid sequence of IABP4 was homologous to that of a yeast protein CTR9, which is known to function, as a subunit of the nuclear complex called PAF1, in transcriptional initiation and elongation, histone modification and control of cell cycle. Therefore, we hypothesized that rice IABP4 also functions by forming a nuclear complex, and that its nuclear localization is regulated by light. In this study, we investigated the intracellular localization of IABP4 and the effect of light on it. First, we examined the intracellular localization of green fluorescent protein (GFP) fusions with full length or various fragments of IABP4 in leaf epidermal cells of onion or Tradescantia virginiana. As a result, we identified one NLS and one NES as functional, respectively. However, the fusion protein with the full length IABP4 was localized both in the nucleus and the cytoplasm. These results suggested that intracellular localization of native IABP4 would change by the balance between nuclear import and export in response to the environmental conditions such as light. So, next we examined whether or not IABP4 will change its intracellular localization in response to light. When the epidermis was incubated under continuous light after particle-bombardment, the nuclear-to-cytoplasmic ratio for the intensity of the fluorescence was decreased, whereas it did not change when the epidermis was incubated in the dark. These results support the idea that IABP4 plays some important roles in light-response of plants throughits light-dependent shuttling between the nucleus and the cytoplasm.