Where do proteins initially enter the secretory pathway?

Proteins initially enter the secretory pathway at the endoplasmic reticulum (ER). The ER serves as the primary site for the synthesis of proteins that are destined for secretion, incorporation into the plasma membrane, or for use in lysosomes. When proteins are synthesized in the ribosomes associated with the rough ER, they are co-translationally translocated into the lumen of the ER.

Once inside the ER, proteins undergo folding and post-translational modifications, such as glycosylation, preparing them for their subsequent transport. After proper folding and modifications, proteins are packaged into transport vesicles that bud off from the ER and head towards the Golgi apparatus for further processing and sorting before they reach their final destinations.

This initial entry point in the ER is crucial because it not only initiates the protein trafficking process but also ensures that only properly folded and assembled proteins continue through the secretory pathway.