They also are responsible for returning the MPR receptors back to the Trans Golgi network. They recycle these by budding off membranes that carry back the receptors and target the Trans Golgi membranes for fusion. After fusion, the MRP receptors are available to capture and sort new degradative enzymes for future trafficking to the late endosomes. Finally, late endosomes may not be able to digest all the material. Therefore, the next step is a fusion of late endosomes and lysosomes creating a hybrid organelle.
LAMPs then become a marker for a late endosome or a lysosome.
Thus, fusion begins after the MPR have been sent back to the Trans Golgi. The steps involved in forming late endosomes and lysosomes are drawn in this cartoon. Note that lysosomes continue to communicate with late endosomes and may deliver important material back to this group of organelles. Lysosomes are considered the end product of endocytosis. However, they could communicate with upstream structures by way of the retrograde transport to the late endosome.