Editing Cisterna (section)

== Secretory pathway ==
The secretory pathway is essential for the sorting, packing, and delivery of proteins to their correct cellular destinations. It begins in the rough endoplasmic reticulum (ER), where proteins are synthesized and initially sorted into vesicles for transport. These vesicles then move to the Golgi apparatus, where they undergo further processing and are directed to their final destinations, such as the plasma membrane, endosomes, or lysosomes.

The first step in the secretory pathway is the formation of transport vesicles at the ER. These vesicles are coated with [[COPII]], a protein complex essential for budding from the ER. COPII coats consist of the small GTP-binding protein Sar1 and two additional complexes: Sec23/Sec24 and Sec13/Sec31. These coat proteins interact with membrane cargo proteins, ensuring that the right proteins are packaged into vesicles. The vesicles then move toward the cis-Golgi network, where they enter via COPII-mediated transport.<ref name=":3" />

The secretory pathway also requires retrograde transport to maintain cellular function. Many ER-resident proteins have specific sorting signals that direct them to be retained in the ER or returned from the Golgi if missorted. This is achieved by [[COPI]]-coated vesicles, which transport these proteins back to the ER from the Golgi in a process called retrograde trafficking. COPI vesicles also play a key role in the movement of Golgi-resident enzymes between different Golgi compartments, ensuring that each compartment maintains the necessary enzymes for proper modification of cargo proteins.<ref name=":3" />

Once vesicles reach the Golgi, they undergo further modifications, including glycosylation and proteolytic processing.<ref name=":3" /> Cargo proteins move through the Golgi compartments (cis, medial, and trans) by either vesicular transport or cisternal maturation. Vesicular transport suggests that the Golgi cisternae remain static while vesicles transport cargo between compartments. In contrast, cisternal maturation poses that the Golgi cisternae themselves mature as enzymes and cargo are progressively moved through the stack, while the cisternae retrogradely exchange enzymes by COPI vesicles.<ref name=":2" />

At the trans-Golgi network, cargo proteins are sorted into different vesicles for delivery to their final destinations. For lysosomal proteins, a crucial modification occurs: the addition of [[mannose-6-phosphate]] (M6P) residues in the cis-Golgi. These M6P tags are recognized by [[mannose-6-phosphate receptor|M6P receptors]] in the trans-Golgi membrane, which directs the proteins toward late endosomes. In the endosomes, the M6P receptors dissociate from the cargo, and the receptors are recycled back to the Golgi or plasma membrane. The lysosomal enzymes are then delivered to the lysosomes for their final role in cellular degradation.<ref name=":3" />

Thus, the secretory pathway is a highly coordinated process, involving various vesicular transport mechanisms and modifications, to ensure that proteins are correctly sorted, processed, and delivered to their appropriate cellular locations.