LAUSR

Abstract ULK serves as an autophagic inductor and plays crucial roles in the formation of autophagy in mammals. However, the function of the ULK counterpart in the autophagy of echinoderms remains ambiguous. In this study, a novel ULK homolog from the sea cucumber Apostichopus japonicus (AjULK) was cloned using RACE technology, and its biological function was investigated. AjULK encoded a predicted protein containing a DUF3543 (746–940 amino acids) domain and a conserved serine/threonine protein kinase (13–277 amino acids) domain. AjULK was widely expressed in all examined organizations, with the largest transcription found in the tentacle. After the Vibrio splendidus challenge in vivo, the mRNA levels of AjULK and other autophagy-related genes, namely, AjAtg13, AjBeclin-1, and AjLC3, were significantly upregulated. By contrast, the level of Ajp62, which was an autophagy substrate, was notably decreased under the same condition. Subcellular localization indicated that AjULK and AjAtg13 were colocalized in the cytoplasm, and the signal was significantly increased after V. splendidus infection. Result indicated that autophagy was markedly induced after pathogen infection. Further functional analysis showed that the protein levels of AjAtg13 and AjBeclin-1 were synchronously decreased after AjULK knockdown, compared with that of the control group. The protein level of Ajp62 was markedly upregulated under the same condition. Thus, the autophagy signal was significantly inhibited through AjBeclin-1 after AjULK interference. Bacterial clearance was also markedly inhibited after AjULK interference. Hence, AjULK served as an autophagic initiator by targeting AjBeclin-1 and mediating the autophagy of coelomocytes to restrict bacterial invasion in the sea cucumber.