Maturation of Endosomes

During maturation, an early endosome moves from the cell periphery toward the perinuclear region along microtubules. Its tubular domains are lost, and the vacuolar domain becomes roughly circular. The endosomal membrane buds inward to form intralumenal vesicles and multivesicular bodies, which contain multiple internal compartments for sorting and degrading internalized molecules.

V-type ATPases pump protons from the cytosol into the maturing endosome's lumen, lowering the intraluminal pH. This acidic environment activates acid hydrolases and enables efficient sorting of internalized molecules. The lower pH provides optimal conditions for degradative enzymes to function properly within the developing late endosome.

SNAREs are tethering proteins delivered to maturing endosomes by the trans-Golgi network. These proteins enable multiple endosomes to fuse with each other, increasing the size and number of intraluminal vesicles in the developing late endosome. SNAREs also facilitate fusion between late endosomes and lysosomes to form endolysosomes.

Late endosomes are called multivesicular bodies because their endosomal membrane buds inward to form numerous intraluminal vesicles. These internal vesicles accumulate as the endosome matures, creating a compartment with multiple membrane-bound structures inside. This morphological feature distinguishes late endosomes from early endosomes, which lack these internal vesicles.

When a late endosome fuses with a lysosome, an endolysosome forms. The acidic environment of the endolysosome maintains the activity of acid hydrolases, which actively degrade endocytosed ligands with the aid of lysosomal enzymes. This fusion represents the final stage of endosomal maturation and enables complete degradation of internalized material.

Lysosomal hydrolases are delivered to maturing endosomes by the trans-Golgi network. These proteins initially contain inhibitory domains that inactivate their function. As maturation progresses, these inhibitory domains are removed, allowing the hydrolases to become active. The acidic pH created by V-type ATPases further supports their degradative activity.

Dynein and kinesin motor proteins mediate endosome movement along microtubules from the cell periphery toward the perinuclear region. Disruption of microtubules or inhibition of dynein delays endosome maturation or disperses late endosomes. These motor proteins are essential for proper spatial positioning of endosomes during their developmental progression.