The Golgi complex sorts the lysosomal enzyme in the Trans region. It is received from the rough endoplasmic reticulum (RER in this cartoon) in the cis region.
There it has a phosphate radical attached to the mannose residue. This mannose-6 phosphate forms a sorting signal that moves through the cisternae to the trans region where it binds to a specific receptor. After it binds to the receptor, it begins to bud and a "cage" or "coat" made of clathrin forms around the bud (to strengthen it). It moves away to fuse with a developing lysosome (such as the vacuoles seen in the previous figure). This lysosome contains a hydrogen ion pump on its surface. The pump works to acidify the environment inside the lysosome. This removes the phosphate and dissociates the hydrolase from the receptor. The receptor is then recycled back to the Golgi complex. Lysosomes can actually be detected by pH indicator dyes. This photograph shows dyes that indicate different pH's with different colors. The red lysosomes (pH 5.0) are probably typical lysosomes. The blue and green lysosomes are probably endosomes. This change can be detected if you link a ligand to fluorescein. Fluorescein will not fluoresce at pH's lower than 6.0. Therefore, one can follow entry of the receptor-ligand complex and then see the fluorescence disappear as the endosome containing the complex is acidified.
