The following images attempt to illustrate
the major complexes and events involved in the modification of proteins that travel through the Golgi apparatus and on to the endosome. Clicking on each of the
thumbnail images will bring up a larger, labeled version of the
see the Flash movie for the following sequence of images,
Lysosomal enzymes, called hydrolases, are first modified in the rough endoplasmic reticulum (ER).
The hydrolases are packaged into vesicles and sent to the cis-cisterna of the Golgi apparatus.
Inside the cis-cisterna, the core sugar chain that was added to the hydrolase in the rough ER is modified again, to create a mannose 6-phosphate (M6P) signal.
Once they reach the trans-Golgi, the M6P signals on the hydrolase ezymes bind to M6P receptors embedded in the trans-Golgi membrane.
Next, a vesicle containing the receptors and hydrolases buds off the trans-Golgi and migrates towards the endosome. Proteins with signals other than M6P are targeted to other locations.
The vesicle docks with the edosome and the receptors release the hydrolases into the endosome.
Inside the endosome the phosphate portion of the M6P signal is removed.
The M6P receptors are then recycled back to the trans-Golgi and the hydrolases are left behind to break down molecules in the lysosome.