Yeast studies identified two heterohexameric tethering complexes, which consist of 4 shared (Vps11, Vps16, Vps18 and Vps33) and 2 specific subunits: Vps3 and Vps8 (CORVET) versus Vps39 and Vps41 (HOPS). CORVET is an early and HOPS is a late endosomal tether. The function of HOPS is well known in animal cells, while CORVET is poorly characterized. Here we show that Drosophila Vps8 is highly expressed in hemocytes and nephrocytes, and localizes to early endosomes despite the lack of a clear Vps3 homolog. We find that Vps8 forms a complex and acts together with Vps16A, Dor/Vps18 and Car/Vps33A, and loss of any of these proteins leads to fragmentation of endosomes. Surprisingly, Vps11 deletion causes enlargement of endosomes, similar to loss of the HOPS-specific subunits Vps39 and Lt/Vps41. We thus identify a 4 subunit-containing miniCORVET complex as an unconventional early endosomal tether in Drosophila.

Two related multisubunit tethering complexes promote endolysosomal trafficking in all eukaryotes: Rab5-binding CORVET that was suggested to transform into Rab7-binding HOPS. We have previously identified miniCORVET, containing Drosophila Vps8 and three shared core proteins, which are required for endosome maturation upstream of HOPS in highly endocytic cells (Lőrincz et al., 2016a). Here, we show that Vps8 overexpression inhibits HOPS-dependent trafficking routes including late endosome maturation, autophagosome-lysosome fusion, crinophagy and lysosome-related organelle formation. Mechanistically, Vps8 overexpression abolishes the late endosomal localization of HOPS-specific Vps41/Lt and prevents HOPS assembly. Proper ratio of Vps8 to Vps41 is thus critical because Vps8 negatively regulates HOPS by outcompeting Vps41. Endosomal recruitment of miniCORVET- or HOPS-specific subunits requires proper complex assembly, and Vps8/miniCORVET is dispensable for autophagy, crinophagy and lysosomal biogenesis. These data together indicate the recruitment of these complexes to target membranes independent of each other in Drosophila, rather than their transformation during vesicle maturation.

Autophagy, the process of cellular self-degradation, is intrinsically tied to the degradative function of the lysosome. Several diseases have been linked to lysosomal degradative defects, including rare lysosomal storage disorders and neurodegenerative diseases. Ion channels and pumps play a major regulatory role in autophagy. Importantly, calcium signaling produced by TRPML1 (transient receptor potential cation channel, mucolipin subfamily) has been shown to regulate autophagic progression through biogenesis of autophagic-lysosomal organelles, activation of mTORC1 (mechanistic target of rapamycin complex 1) and degradation of autophagic cargo. ER calcium channels such as IP₃Rs supply calcium for the lysosome, and lysosomal function is severely disrupted in the absence of lysosomal calcium replenishment by the ER. TRPML1 function is also regulated by LC3 (microtubule-associated protein light chain 3) and mTORC1, two critical components of the autophagic network. Here we provide an overview of the current knowledge about ion channels and pumps—including lysosomal V-ATPase (vacuolar proton-ATPase), which is required for acidification and hence proper enzymatic activity of lysosomal hydrolases—in the regulation of autophagy, and discuss how functional impairment of some of these leads to diseases.

In this review, we present recent scientific advances about integrin trafficking in the endo-lysosomal system. In the last few years, plenty of new information has emerged about the endo-lysosomal system, integrins, and the mechanism, how exactly the intracellular trafficking of integrins is regulated. We review the internalization and recycling pathways of integrins, and we provide information about the possible ways of lysosomal degradation through the endosomal and autophagic system. The regulation of integrin internalization and recycling proved to be a complex process worth studying. Trafficking of integrins, together with the regulation of their gene expression, defines cellular adhesion and cellular migration through bidirectional signalization and ligand binding. Thus, any malfunction in this system can potentially (but not necessarily) lead to tumorigenesis or metastasis. Hence, extensive examinations of integrins in the endo-lysosomal system raise the possibility to identify potential new medical targets. Furthermore, this knowledge can also serve as a basis for further determination of integrin signaling- and adhesion-related processes.

Proper functioning of the precisely controlled endolysosomal system is essential for maintaining the homeostasis of the entire cell. Tethering factors play pivotal roles in mediating the fusion of different transport vesicles, such as endosomes or autophagosomes with each other or with lysosomes. In this work, we uncover several new interactions between the endolysosomal tethering factors Rabenosyn-5 (Rbsn) and the HOPS and CORVET complexes. We find that Rbsn binds to the HOPS/CORVET complexes mainly via their shared subunit Vps18 and we mapped this interaction to the 773-854 region of Vps18. Based on genetic rescue experiments, the binding between Rbsn and Vps18 is required for endosomal transport and is dispensable for autophagy. Moreover, Vps18 seems to be important for β1 integrin recycling by binding to Rbsn and its known partner Vps45.

The closely related endolysosomal tethering complexes HOPS and CORVET play pivotal roles in the homo- and heterotypic fusion of early and late endosomes, respectively, and HOPS also mediates the fusion of lysosomes with incoming vesicles including late endosomes and autophagosomes. These heterohexameric complexes share their four core subunits that assemble with additional two, complex-specific subunits. These features and the similar structure of the complexes could allow the formation of hybrid complexes, and the complex specific subunits may compete for binding to the core. Indeed, our biochemical analyses revealed the overlap of binding sites for HOPS-specific VPS41 and CORVET-specific VPS8 on the shared core subunit VPS18. We found that the overexpression of CORVET-specific VPS8 or Tgfbrap1 decreased the amount of core proteins VPS11 and VPS18 that are assembled with HOPS-specific subunits VPS41 or VPS39, indicating reduced amount of assembled HOPS. In line with this, we observed the elevation of both lipidated, autophagosome-associated LC3 protein and the autophagic cargo p62 in these cells, suggesting impaired autophagosome-lysosome fusion. In contrast, overexpression of HOPS-specific VPS39 or VPS41 did not affect the level of assembled CORVET or autophagy. VPS8 or Tgfbrap1 overexpression also induced Cathepsin D accumulation, suggesting that HOPS-dependent biosynthetic delivery of lysosomal hydrolases is perturbed, too. These indicate that CORVET-specific subunit levels fine-tune HOPS assembly and activity in vivo.