Cell Biology: Membrane Trafficking Overview

Membrane Trafficking Overview

• Membrane trafficking is a critical cellular process for maintaining cell function
• It transports nutrients and other solutes to all parts of the cell
• It involves a complex, highly regulated system like a parcel sorting office
• The transport can occur within organelles or across the cell membrane
• This process is divided into two pathways based on the direction of travel: endocytic pathway and secretory pathway

Endoplasmic Reticulum (ER)

• The ER plays a role in protein secretion
• It targets proteins to the ER
• Proteins are inserted into the ER membrane
• Proteins and lipids are exported from the ER

Golgi Apparatus

• The Golgi apparatus organizes proteins within the Golgi
• Protein glycosylation occurs within the Golgi
• The Golgi handles lipid and polysaccharide metabolism
• Proteins are sorted and exported from the Golgi apparatus

Vesicular Transport

• Cargo selection, coat proteins, and vesicle budding are involved in vesicular transport
• Vesicle fusion is a critical step
• Membrane trafficking involves the movement of cargo (proteins, pathogens, and other macro-molecules) using membrane-bound transport vesicles

Lysosomes

• Lysosomal acid hydrolases are present in lysosomes
• Endocytosis and lysosome formation occur in lysosomes
• Phagocytosis and autophagy are processes that happen in lysosomes

Protein Trafficking

• Proteins must be sent to the correct location after synthesis
• Transport vesicles are involved in mediating membrane trafficking
• Protein trafficking is divided into vesicle-mediated and non-vesicle mediated traffic types

Protein sorting

• Proteins synthesized in free ribosomes in the cytosol can remain in the cytosol or be transported to the nucleus, mitochondria, chloroplasts, or peroxisomes.
• Proteins synthesized from membrane-bound ribosomes are translocated into the ER during translation and then either remain in the ER, or are transported to the Golgi apparatus, lysosomes, the plasma membrane, or the cell exterior via secretory vesicles.

Secretory Pathway

• The secretory pathway involves transporting proteins from the rough ER to the Golgi to secretory vesicles to the cell exterior
• The ER and Golgi are essential as protein modification

Protein Retrieval

• Proteins destined to remain in the ER lumen are marked with the KDEL sequence
• These proteins are exported from the ER to the Golgi, but the Golgi and ERGIC receptors will ensure return to the ER
• Proteins are sorted in the trans Golgi network to their final destinations

Golgi Organization

• The Golgi apparatus consists of flattened membrane-enclosed sacs receiving proteins from the ER to process and sort proteins to reach their eventual destinations

Golgi Regions

• Vesicles from the ER fuse to form the ER-Golgi intermediate compartment
• Proteins are transported from the ER-Golgi intermediate compartment to the cis Golgi network
• Proteins are returned back to the ER or Golgi via recycling pathway
• Final modifications occur in the medial and trans compartments of the Golgi stack

Transport from the Golgi Apparatus

• Proteins are sorted in the trans Golgi network
• Proteins can be carried to the plasma membrane via constitutive secretion
• Proteins can also be transported from the Golgi network via a regulated pathway to target other locations such as endosomes or lysosomes

Vesicle Fusion

• Vesicle fusion involves two events: the transport vesicle recognizing its target membrane and the vesicle and target membranes fusing together
• Coat proteins help sort the proteins for transport into the coated bud for transport
• SNARE proteins drive membrane fusion

Coat proteins

• Coat proteins are involved in cargo selection, vesicle budding
• Different kinds of coat proteins are involved in various transport steps