Membrane Trafficking Overview

Membrane Trafficking Overview

• Membrane trafficking is a vital cellular process maintaining cell sustainability
• It transports nutrients and other solutes to all parts of the cellular system
• It involves a range of processes using membrane bound transport vesicles
• It's a complex, highly regulated system akin to a parcel sorting office
• Transport occurs within organelles or across the cell membrane to/from the extracellular environment
• Divided into two basic pathways: Endocytic and Secretory pathways, based on the direction of travel

Endocytic Pathway/Endocytosis

• Endocytosis is the movement of cargo into the cell from the plasma membrane
• It's used for uptake of nutrients the cell can't synthesize (vitamins, cholesterol, iron)
• Involves the movement of cargo to the plasma membrane or out of the cell as part of the biosynthetic-secretory pathway

Secretory Pathway/Exocytosis

• Exocytosis is the movement of cargo to the plasma membrane or out of the cell
• Cargo moves via the Golgi from the endoplasmic reticulum (ER)
• It's part of the biosynthetic-secretory pathway

Protein Trafficking

• Proteins must be sent to their correct cellular locations after synthesis
• Transport vesicles mediate this membrane trafficking
• Mechanisms are categorized as vesicle-mediated (ER ribosomes) and non-vesicle mediated (free or cytosolic ribosomes)

Overview of Protein Sorting

• Proteins synthesized on free ribosomes in the cytosol either remain in the cytosol or are transported to structures like the nucleus, mitochondria, chloroplasts, or peroxisomes
• Proteins synthesized on membrane-bound ribosomes are translocated into the ER during translation
• Proteins then travel via the ER through the Golgi before to the plasma membrane or other locations (lysosomes)

Secretory Pathway Detail

• The rough ER→Golgi→secretory vesicles→cell exterior pathway is a key element in the secretory pathway
• Proteins targeted for secretion, incorporation into the ER/Golgi/lysosomes/plasma membrane are initially targeted to the ER

Distributions of Coat Proteins

• ER export sites (ERES)
• Vesicular tubular carriers (VTCs)
• Coat proteins use different pathways (Clathrin, COPI, COPII)

Overview of the Secretory Membrane System

• Transport carriers move lipids and proteins between organelles like ER, Golgi, and plasma membrane
• Transport between organelles is carried out by small vesicles/ larger vesicle-tubule elements that move along microtubules
• Carriers form at ER export sites (ERES) and become Vesicular tubular carriers (VTCs) to the Golgi

Endoplasmic Reticulum, Golgi, Endosomes, and Lysosomes

• These organelles work together, largely through vesicular transport, to process and modify proteins
• Their unique functions and protein involvement distinguishes them from other cytoplasmic organelles

The Anterograde Pathway

• Newly synthesized proteins travel from the ER to Golgi to the plasma membrane, or endosome/lysosomal system

The Retrograde Pathway

• Retrograde transport carriers return protein and lipids from the VTC/Golgi back to the ER
• Different carriers move proteins/lipids to trans-Golgi network (TGN)

Protein Machinery for Secretory Transport

• Small carriers (motor proteins, tethers, Rab GTPases/ tethers)
• Large carriers (SNARES)
• Cargo proteins move through the secretory system, using various vesicle and protein mechanisms

Vesicle Fusion

• Fusion of transport vesicles with target membranes involves two important events
• Recognition of target membrane (e.g., lysosomes) by vesicle
• Fusion of vesicle and target membrane to deliver cargo to the target organelle

Cargo Selection, Coat Proteins, and Vesicle Budding

• Coated vesicles with coat proteins (COPI, COPII, Clathrin) carry secretory proteins from ER to Golgi (and other targets)
• Key for selection and transport

Formation and Fusion of a Transport Vesicle

• Membrane proteins and other relevant cargo are collected into selected donor membrane regions.
• Coating results in transport vesicle budding off
• Coats are disassembled after transport to allow fusion with the target membrane

Regions of the Golgi Apparatus

• Vesicles from the ER fuse to form ER-Golgi intermediate compartment (ERGIC)
• Proteins from ER are transported to the cis-Golgi network
• Resident ER proteins are returned from Golgi networks via recycling pathway
• Golgi stacks (medial and trans compartments) are sites of key protein modifications and protein sorting to the trans-Golgi network

Transport from the Golgi Apparatus

• Proteins are sorted within trans-Golgi network and directed to final destinations
• Proteins are carried to the plasma membrane via constitutive secretion, or to alternative destinations like endosomes and lysosomes via regulated secretion

Explore the fundamental processes of membrane trafficking, which is crucial for cellular sustainability. This quiz dives into endocytosis and exocytosis pathways, highlighting their roles in nutrient transport and cell communication. Test your understanding of these complex mechanisms and their significance in cellular function.