The hookup model of the HOPS complex in autophagosome-lysosome fusion

Macroautophagy/autophagy is a highly conserved process that involves the degradation of proteins, damaged organelles, and other cytoplasmic macromolecules. Autophagosome-lysosome fusion is critical for successful substrate degradation and is mediated by SNARE proteins. The fusion process requires additional vesicle docking and tethering-regulating factors. Our recent work has uncovered a functional model of autophagosome-lysosome fusion. We demonstrated that the six-subunit homotypic fusion and vacuole protein sorting (HOPS) complex can be assembled by two subcomplexes, the VPS39-VPS11 subcomplex (HOPS-2) and the VPS41-VPS16-VPS18-VPS33A subcomplex (HOPS-4). VPS39 binds with RAB2 on the autophagosome and VPS41 binds with RAB39A on the lysosome, which then promotes membrane tethering and autophagic SNARE-mediated membrane fusion. Moreover, we have revealed that ALS- and FTD-related C9orf72 is a guanine exchange factor (GEF) for RAB39A. In this punctum, we discuss how the C9orf72-RAB39A-HOPS axis function regulates autophagosome-lysosome fusion.

Prominent role of RAB39A-RXRB axis in cancer development and stemness

In this study, we found that RAB39A, a member of the RAS oncogene family, was selectively expressed in cancer cells of different histotypes, by analyzing gene expression in human osteosarcoma cells and the cancer stem cells (CSCs) and by comparing them with normal cells through global transcriptomics and principal component analyses. We further validated RAB39A as a therapeutic target, by silencing its expression. The silencing impaired cancer stemness and spherogenic ability in vitro, as well as tumorigenesis in vivo. RNA-seq analyses in the silenced spheres suggested that RAB39A is associated downstream with RXRB and KLF4. Notably, RXRB expression was inhibited in RAB39A-silenced CSCs. Induced overexpression of RXRB in RAB39A-silenced cells restored spherogenic ability and tumorigenesis, confirming RXRB as a major effector of RAB39A. Quantitative RT-PCR analysis of ∼400 human cancer tissues showed that RAB39A was highly expressed in sarcomas and in malignancies of lymphoid, adrenal and testicular tissues. Our data provide the rationale for targeting of the RAB39A-RXRB axis as a therapy for aggressive cancers.