腺嘌呤核苷二磷酸核醣化相似因子 (ARF-like protein 1, ARL1) 屬於腺嘌呤核苷二磷酸核醣化因子家族的成員之一 (ADP-ribosylation factors, ARFs)。在真核生物中,這類蛋白被報導參與許多重要的細胞功能,包含調控囊泡運輸、影響細胞骨架的組裝以及維持細胞內的離子恆定等等,但是作用機制的細節仍不甚清楚。在酵母菌的研究當中,已知鳥糞嘌呤核苷酸交換因子 (Guanine nucleotide exchange factor) Syt1p 會催化 Arl1p 的活性並使其前往反式高基氏體網路 (trans-Golgi network)。活化態的 Arl1p 將吸引高基氏體蛋白 (Imh1p) 至反式高基氏體網路並作用於下游的囊泡運輸。在我們先前的發表成果中,我們發現內質網壓力 (ER stress) 的誘發將促使內質網上的磷酸化激酶 Ire1p 活性上升,Ire1p將透過 Syt1p 的第416號絲氨酸磷酸化轉譯後修飾進一步大量活化 Arl1p。此外,我們也證實這條路徑的啟動有助於細胞抵擋內質網壓力。而在本篇研究中,我們發現在細胞經歷內質網壓力的過程中, Hog1p 是另一個調控 Syt1p的磷酸化激酶。不同於 Ire1p 的是,Hog1p 作用於 Syt1p 的第297號絲氨酸磷酸化轉譯後修飾以活化 Arl1p。同時,我們亦發現不受壓力激活的 Hog1p 也可以調控 Arl1p 的活化,說明在壓力環境以及正常生理狀態下,Hog1p 皆扮演活化 Arl1p 的重要角色。總結本篇除了再次說明 Syt1p 的磷酸化轉譯後修飾對於 Arl1p 活化的重要性,我們亦提出 Hog1p 在正常生理狀態中所執行的新功能。
ARF-like (ARL) proteins play important roles in regulating vesicular trafficking at the Golgi compartments, modulating cytoskeleton dynamics and maintaining ion homeostasis. In Saccharomyces cerevisiae, guanine-nucleotide exchange factor (GEF) Syt1p facilitates Arl1p activation to recruit golgin protein Imh1p to trans-Golgi network (TGN). Our previous findings have elucidated the induction of unfolded protein response (UPR) highly promotes activation of Arl1p and that ER-resident kinase Ire1p is responsible for the up-regulation through transcription alteration (Proc. Natl. Acad. Sci. U S A. 113:E1683-E1690). Moreover, the SYT1-ARL1-IMH1 signaling is required for ER stress resistance. In this study, we identified MAP kinase Hog1p as a new regulator of the ARL1 pathway for UPR. We found that MAP kinase controls Arl1p activation and non-stressed Hog1p mutant can rescue Imh1p Golgi localization in MAP kinase-deleted cells. We further showed that Hog1p might directly phosphorylate Syt1p at serine 297 to promote the activation of Arl1p, which performs a distinct route from Ire1p-mediated Syt1p phosphorylation at serine 416. Collectively, we demonstrate that Syt1p phosphorylation is regulated by multi-regulators, which is important for the activation and function of Arl1p under stress as well as normal growth conditions.