GO:0061763 multivesicular body-lysosome fusion

Multivesicular body-lysosome fusion (GO:0061763) is the organelle membrane fusion process in which the membrane of a multivesicular body (MVB) fuses with a lysosome to create a hybrid organelle. This event represents the terminal delivery step of the endolysosomal pathway, allowing the contents of MVBs, including intraluminal vesicles and their cargo, to be transferred into the lysosome for degradation and downstream signaling. Because it controls the fate of a large fraction of internalized and sorted membrane proteins, MVB-lysosome fusion is central to cellular homeostasis. For researchers, GO:0061763 matters because it is mechanistically distinct from other endolysosomal fusion events, such as homotypic endosome-endosome fusion or autophagosome-lysosome fusion. A dedicated cell-free content-mixing assay has been developed that specifically reports MVB-lysosome fusion, providing a biochemical handle on this process. This assay has revealed distinct features of MVB-lysosome fusion, including its requirements and kinetics, that differentiate it from related membrane fusion reactions. Understanding GO:0061763 is therefore essential for interpreting endolysosomal trafficking phenotypes, for dissecting how cargo is delivered to lysosomes, and for modeling diseases in which lysosomal degradation or extracellular vesicle release is perturbed. The term provides a precise ontology anchor for annotating genes and experiments that specifically act at the MVB-lysosome fusion step.

GO:0061763, multivesicular body-lysosome fusion, is defined as the organelle membrane fusion process in which the membrane of a multivesicular body fuses with a lysosome to create a hybrid organelle. In other words, it is the specific membrane merger event that joins an MVB (also called a late endosome) to a lysosome, forming a hybrid organelle that mixes the contents of both compartments. This definition distinguishes it from other fusion processes in the endolysosomal system, such as endosome-endosome fusion or autophagosome-lysosome fusion.

MVB-lysosome fusion is important because it is the committed step that delivers multivesicular body cargo, including intraluminal vesicles and their protein and lipid contents, into the lysosome for degradation and signaling. Without this fusion event, endocytosed and sorted cargo cannot reach the lysosomal lumen, and the cell cannot complete the endolysosomal degradation pathway. Because a dedicated cell-free content-mixing assay can specifically measure this fusion, the process is experimentally tractable and can be dissected biochemically, which is essential for assigning gene function to the correct fusion step.

The genes and proteins below are the core machinery and regulatory components associated with multivesicular body-lysosome fusion (GO:0061763), as studied using the cell-free content-mixing assay and related endolysosomal approaches.

MVB-lysosome fusion is regulated at the level of membrane recognition, docking and merger, and can be studied biochemically using a cell-free content-mixing assay that specifically reports this fusion event. The distinct features of MVB-lysosome fusion revealed by this assay indicate that the process has its own regulatory requirements, separate from other endolysosomal fusion reactions. Because the assay measures content mixing directly, it can be used to test how specific gene products and conditions regulate GO:0061763.

Because GO:0061763 is the terminal delivery step of the endolysosomal pathway, defects in this fusion event are expected to impair lysosomal degradation and cause accumulation of undegraded cargo. The cell-free content-mixing assay provides a way to test whether disease-associated perturbations specifically affect MVB-lysosome fusion rather than upstream MVB biogenesis or downstream lysosomal function.

MVBs are the source of exosomes, so the balance between MVB-lysosome fusion and MVB secretion at the plasma membrane influences extracellular vesicle release. Assays that specifically measure MVB-lysosome fusion, such as the cell-free content-mixing assay, help determine how changes in this fusion step alter the fate of MVB cargo.

Altered endolysosomal trafficking, including MVB-lysosome fusion, has been linked to cancer and neurodegenerative disease contexts in which lysosomal degradation or vesicle release is perturbed. Using assays that report GO:0061763 directly allows researchers to test whether candidate disease genes act at this specific fusion step.

The cell-free content-mixing assay is the key method for studying GO:0061763 because it specifically reports MVB-lysosome fusion and has revealed distinct features of this process. It allows the fusion event to be reconstituted and measured biochemically, separating it from other endolysosomal fusion reactions.

Fluorescence imaging of MVB markers such as CD63 and lysosome markers such as LAMP1 can be used to monitor the formation of hybrid organelles after MVB-lysosome fusion. Tagged knock-in reporters enable tracking of cargo delivery from MVBs to lysosomes.

CRISPR knockout, point-mutation, knock-in and overexpression models can be combined with the content-mixing assay to determine whether a specific gene is required for MVB-lysosome fusion. This approach helps assign gene function to the correct step of the endolysosomal pathway.

Because the content-mixing assay is cell-free, it can be used to test the biochemical requirements and distinct features of MVB-lysosome fusion. Such experiments help define the molecular mechanism of GO:0061763 and distinguish it from related fusion events.

Researchers studying multivesicular body-lysosome fusion-related genes often need to determine whether a candidate gene is causally involved in this specific membrane fusion event, rather than in upstream MVB biogenesis or downstream lysosomal function. Because GO:0061763 can be measured directly with a cell-free content-mixing assay, pairing that assay with precise CRISPR-engineered cell models provides a rigorous way to assign gene function to this step.

GO:0061763, multivesicular body-lysosome fusion, is the specific membrane fusion event that joins a multivesicular body to a lysosome to form a hybrid organelle, completing the terminal delivery step of the endolysosomal pathway. Its distinct features can be measured directly with a cell-free content-mixing assay, making it experimentally separable from other endolysosomal fusion events. For researchers, precise annotation and assay choice are essential: genes should be tested for causal roles at the fusion step itself, not merely for effects on upstream MVB biogenesis or downstream lysosomal function. Combining fusion-specific assays with CRISPR knockout, point-mutation, knock-in and overexpression models provides a rigorous framework for dissecting the molecular mechanism and disease relevance of MVB-lysosome fusion.