在阿拉伯芥中，EXPORTIN1A (HIT2/XPO1A)和EXPORTIN1B (XPO1B)負責把帶有NES的蛋白質從細胞核運送到細胞質。然而，HIT2/XPO1A突變而非XPO1B突變會引發強光敏感。由於阿拉伯芥熱逆境因子A4a和A5 (AtHsfA4a and AtHsfA5)參予在強光逆境反應中並且帶有NES，因此我們分析其細胞核質分布。在野生型和xpo1b細胞，AtHsfA4a常態分佈在細胞質但是隨著強光處裡卻變成在細胞核，而AtHsfA5則持續分布在細胞質和細胞核。然而在hit2/xpo1a細胞，不管有無強光處裡，AtHsfA4a和AtHsfA5都會分布在細胞核。雖然AtHsfA4a可以強化植物清除H2O2的能力，並且AtHsfA5是AtHsfA4a的repressor，但是athsfa5突變株而非athsfa4a突變株會對強光敏感。此外，表現AtHsfA4a∆NES的athsfa4a突變株是對於強光敏感，但是表現AtHsfA5∆NES的athsfa5突變株是對於強光不敏感。同時，hit2/athsfa4a雙重突變株比起hit2是較耐受強光逆境。這些結果顯示，AtHsfA4a和AtHsfA5都是HIT2/XPO1A的專一受質。AtHsfA4a長期的累積會造成hit2的強光敏感性狀，並且這是獨立於其清除H2O2的能力，AtHsfA5的存在則可以減輕這不利的影響。雖然AtHsfA4a的細胞核質分布也受熱處理改變，但是持續性高溫逆境和熱休克逆境分析顯示，AtHsfA4a和AtHsfA5的調控機制不參與在hit2的熱敏感性狀中。;In Arabidopsis, EXPORTIN1A (HIT2/XPO1A) and EXPORTIN1B (XPO1B) mediate the translocation of nuclear export sequence (NES)-bearing proteins from nucleus to cytoplasm. However, a mutation in HIT2/XPO1A but not in XPO1B induces sensitivity to high light (HL). Arabidopsis thaliana heat stress factors A4a and A5 (AtHsfA4a and AtHsfA5) are involved in plant responses to HL and possess NESs; therefore, their nucleo-cytoplasmic partitioning was analyzed. In wild-type and xpo1b mutant cells, AtHsfA4a normally remained in the cytoplasm but became concentrated in the nucleus following exposure to HL, whereas AtHsfA5 was constitutively distributed in both cytoplasm and nucleus. However, in hit2/xpo1a mutant, AtHsfA4a and AtHsfA5 were always confined to the nucleus, regardless of the irradiance. Although AtHsfA4a can enhance the ability of plants to scavenge H2O2, and AtHsfA5 is a repressor of AtHsfA4a, athsfa5 but not athsfa4a mutant plants exhibited HL sensitivity. Additionally, athsfa4a plants expressing AtHsfA4a∆NES were sensitive to HL, but athsfa5 plants expressing AtHsfA5∆NES were not. Meanwhile, hit2/athsfa4a double mutant was more tolerant to HL than hit2. These results indicate that both AtHsfA4a and AtHsfA5 were HIT2/XPO1A-specific substrates. Long-term accumulation of AtHsfA4a contributed to the hit2 HL-sensitive phenotype independent of the scavenging ability of H2O2, and the presence of AtHsfA5 could mitigate this adverse effect. Although nucleo-cytoplasmic distribution of AtHsfA4a was heat-mediated changed, analysis of sustained heat stress and heat shock showed that the regulation of AtHsfA4a and AtHsfA5 is independent of heat-sensitive phenotype of hit2.
