Transport vesicles mediate through two pathways:

  1. Outward Secretory Pathway (goes out through the cell surface); normally includes translocation across the ER followed by transfer to stacked Golgi cisternae (interconnected network of membranes also known as cis-Golgi), then to the trans-Golgi network where the protein will bud-off in a transport vesicle created by the trans-Golgi membrane, a process called exocytosis. 

Outward Secretory Pathway. Click on image for credit.

Cis and trans refers to the different faces of the Golgi complex. Vesicles come into the cis face from the RER and leave from the trans face to the plasma membrane or lysosomes.

Each molecule that travels along this route passes through a fixed sequence of modifications in the membrane and lumen of the RER and Golgi complex;

  • Formation of disulfide bonds– Disulfide bonds (composed of two sulfur atoms united to carbon atoms. More of in biochemistry) help stabilize the structure of proteins. They do not form in the cytosol because of the reducing (electron giving and therefore disrupting the bond) environment there that interrupts the formation of disulfide bonds.
  • Addition and processing of carbohydrates (glycosylation as in –ation or addition of glycose)- Glycosylation can protect the protein from degradation, and also to hold it in the ER until it is properly folded, or help guide it to the appropriate organelle by serving as a transport signal in the transport vesicle (because it can function in recognition).

Example of an N-Linked Glycosylation. Click on image for credit.

Oligosaccharides (14 sugars) side chains link to an amino acid’s (usually asparagine) amide (-NH2) group, specifically to the nitrogen atom, so it is called N-linked glycosylation.

Modifications to N-linked oligosaccharides are completed in the Golgi complex. The middle region of Golgi apparatus contains enzymes- glycosyl transferases– for this purpose. Some proteins are glycosylated by O-linked oligosaccharides (1-4 sugars) in both the ER and Golgi apparatus.

  • Proper folding– several strategies are used to ensure that only proteins properly folded are transported. Some examples are stress response in the ER are ER associated protein degradation (ERAD) and unfolded protein response (UPR).
  • Specific proteolytic cleavages– assembly into multimeric (different polypeptide chains come together) proteins.

         2. Inward Endocytic Pathway (directed to endosomes or lysosomes within the cell);  Endocytosis is a process by which cells take up and absorb molecules into it from the extracellular matrix (ECM) by engulfing them. Part of the plasma membrane is invaginated during this process and pinches of forming a membrane-bound vesicle called an endosome.

Endocytosis. Click on image for credit.

The endosome exists in two phases: an early endosome whose task is to physically separate or sort receptor from ligands (signals) that have been internalized, this is why early endosomes are called sorting endosomes. The next phase is the late endosome involved in the breakdown of internalized cargo.

The two types of endosomes. Early(Sorting) and late endosomes. Click on image for credit.

There are three forms of endocytosis:

Example of Receptor-Mediated Endocytosis (RME). Click on image for credit.

Receptor sorting is the major form of endosome sorting because the receptors engage in rapid recycling. The other two are clear enough without illustrations I guess. If not, then google them 🙂