Cell Biology: Membrane Trafficking Overview

Membrane Trafficking Overview

• Membrane trafficking is vital for cellular sustainability.
• It transports nutrients and solutes to all cellular components.
• It involves a complex, highly regulated system, analogous to a parcel sorting office.
• This process can happen within organelles or across the cell membrane to the extracellular environment.
• Membrane trafficking occurs along two pathways: Endocytic and Secretory.

Endocytic Pathway

• Endocytosis is the movement of cargo into the cell.
• This process often takes up nutrients that the cell cannot synthesize (e.g., vitamins, cholesterol, iron).
• It involves various stages from early endosome to late endosome and lysosomes.

Secretory Pathway

• Exocytosis is the movement of cargo out of the cell or to the plasma membrane.
• It's part of the biosynthetic-secretory pathway.
• Newly synthesized proteins, lipids, and carbohydrates travel from the ER to the Golgi and then to the cell membrane or extracellular space.

Protein Trafficking

• Proteins must be transported to their correct location after synthesis.
• Transport vesicles are key mediators of membrane trafficking.
• Protein trafficking mechanisms are categorized as vesicle-mediated and non-vesicle-mediated.

Mechanisms of Protein Trafficking

• Vesicle-mediated trafficking involves proteins made in ER ribosomes.
• Non-vesicle-mediated trafficking involves proteins made in free or cytosolic ribosomes.
• Higher eukaryotic cells have an overview of protein sorting.

Protein Sorting in Higher Eukaryotic Cells

• Proteins synthesized on free ribosomes either remain in the cytosol or are transported to the nucleus, mitochondria, chloroplasts, or peroxisomes.
• Proteins synthesized on membrane-bound ribosomes are translocated into the ER while their translation occurs.
• These proteins are either retained in the ER or transported to the Golgi, lysosomes, plasma membrane, or cell exterior via secretory vesicles.

Golgi Apparatus

• The Golgi apparatus gets proteins from the ER, processes them, and sorts them to their final destinations.
• It's composed of flattened membrane-enclosed sacs.

The Secretory Pathway

• Proteins follow the pathway involving Rough ER -> Golgi -> Secretory vesicles -> Cell exterior.
• Proteins destined for secretion or incorporation into ER, Golgi apparatus, lysosomes, or plasma membrane are initially targeted to the ER.
• The ER proteins to remain in the ER are marked by the sequence Lys-Asp-Glu-Leu (KDEL) on their carboxy terminus and will be retrieved back to the ER.

Vesicular Transport

• Vesicular transport between ER and the Golgi involves transition proteins.
• These proteins move along the membrane of the transitional ER via vesicles, fuse with the ER-Golgi intermediate compartment, and are transported to the Golgi.
• Proteins in the lumen are taken up by the vesicle, and released to the Golgi lumen.
• Membrane proteins maintain the same orientation in the Golgi as in the ER.

Transport from Golgi

• Proteins are sorted in the trans Golgi network.
• They are then transported to their destination in vesicles.
• If there are no specific targeting signals, proteins are carried via the plasma membrane (constitutive secretion).
• Proteins can also be targeted to endosomes/lysosomes or undergo regulated secretion.

Mechanism of Vesicular Transport

• Transport vesicles have specific mechanisms and proteins for recognizing their target membrane.
• Vesicle and target membranes fuse, releasing the vesicles' contents into the target organelle.
• Vesicles are coated with coat proteins, which then detach in the cytosol.

Cargo Selection, Coat Proteins, and Vesicle Budding

• Most transport vesicles are coated with cytosolic coat proteins, like COPI, COPII, and clathrin.
• These proteins classify the vesicles.

Formation and Fusion of Transport Vesicles

• Membrane and lumenal proteins assemble into the vesicles, which then bud off from a donor membrane.
• After transport, the coat detaches, and the vesicle docks with the target membrane and fuses.

Vesicle Fusion

• The process of vesicle fusion involves several steps.
• First, the vesicle recognizes its target membrane.
• Second, the vesicle and its target membrane fuse together.

This quiz explores the essential processes of membrane trafficking in cells, including endocytosis and exocytosis. Learn how nutrients and solutes are distributed within cellular compartments and the extracellular environment. Understand the steps involved in both the endocytic and secretory pathways.