Endosomelysosome

Endosomelysosome is a term used to describe the fused compartment that forms when an endosome merges with a lysosome as part of the cellular endocytic pathway. In many texts this compartment is called an endolysosome or lysosome–endosome hybrid. The maturation of endosomes proceeds from early endosomes to late endosomes and ultimately to fusion with lysosomes, creating a degradative environment with low luminal pH.

The endolysosome or endosome–lysosome fusion compartment contains lysosomal hydrolases transferred from the lysosome and membranes that regulate access to the lumen. The acidic pH, maintained by the vacuolar-type H+-ATPase, activates these enzymes to degrade proteins, nucleic acids, lipids, and other internalized material. Degradation products such as amino acids, sugars, and nucleotides are transported back into the cytosol for reuse by specific transporters, while some larger cargos may be repackaged or recycled to the plasma membrane. The fusion process is mediated by SNARE proteins, Rab GTPases, and tethering complexes, coordinating maturation and cargo processing.

Physiological roles include turnover of receptors, disposal of endocytosed material, and antigen processing for MHC class II presentation. Autophagy pathways intersect this system through autolysosome formation, a related degradative compartment. Dysfunctions of endosome–lysosome fusion or lysosomal enzyme delivery are implicated in lysosomal storage disorders and are increasingly recognized in neurodegenerative diseases. The endosomelysosome concept highlights the degradative center at the crossroads of endocytosis, autophagy, and intracellular quality control.