A brief primer on the secretory pathway

academics biology cell biology

The secretory pathway is arguably one of the most important pathways in the body. It deemed ‘Secretory’ primarily because it’s the pathway that controls how the cell secretes proteins int extracellular environment.

This pathway involves a few main players – the Endoplasmic reticulum (ER), Golgi Apparatus and the Lysosome. Let’s dig a bit deeper into the role and functions of each of these organelles below. 

After transcription (DNA --> RNA) in the nucleus and translation (RNA --> protein) in the ribosome, a new protein product is formed. What does this new protein product want to do next? It wants to become biologically active. How does it do so? By folding! What helps fold these proteins? Chaperones! Where does this folding take place? The endoplasmic reticulum! 

Proteins are imported into the ER using a signal sequence (seen on the newly made protein). This sequence is bound by a signal recognition particle (SRP). This creates a SRP – ribosomal protein complex. This SRP-ribosomal protein complex binds to the SRP receptor on the ER. This binding releases SRP and allows our signal sequence to enter the ER. 

The endoplasmic reticulum (rough) can be thought of as the main protein folding and quality control apparatus. It’s here that translated proteins from the ribosome can be twisted and turned to properly function. It’s also where mis/unfolded proteins can be dealt with (sent or tagged for degradation). 

After folding is complete in the ER, our folded protein exits the ER in lipid vesicles and head toward the cis side of the Golgi Apparatus. Here, proteins are modified and packaged into transport vesicles to their final destination (essentially, where they’ll be working in the cell). 

Digestive enzymes (proteins) are packaged and pinched off into a specific type of vesicle. That vesicle is a lysosome! These lysosomes are packed with a digestive punch and work to break down complex molecules inside the cell.  

For more practice, try understand what happens when certain parts of this pathway are disrupted. This will help drive home the interconnectedness of these organelles and larger function. Happy studying!