Endocytosis and Exocytosis

• Cell, Molecular, and Developmental Biology II, Lecture #12 covers intracellular trafficking

Overview

• General principles of intracellular trafficking
• Rab proteins
• ER to Golgi transport
• Transport through the Golgi apparatus
• Endocytosis
• Exocytosis

Introduction

• The cytoplasm may appear chaotic, but directional transport within the cell is tightly regulated
• Vesicles and associated cargo move between organelles and the plasma membrane

Cellular Capture and Release

• Cells capture and release molecules at the plasma membrane
• Exocytosis releases molecules like cytokines into the environment
• Endocytosis removes molecular signals from the extracellular environment

Topology Preservation

• Topology remains consistent during trafficking
• The outside of the cell becomes the inside (lumen) of a vesicle during endocytosis
• When a vesicle merges with its target, its inside (lumen) releases contents into the organelle's inside (lumen)

Coated Vesicles

• Protein coats aid vesicles in cargo packaging and budding from their origin membrane
• Clathrin, COPI, COPII, and retromer are types of coats

Clathrin Coat Assembly

• Clathrin coats primarily assemble on the plasma membrane's inner surface during endocytosis
• Clathrin coats have been observed by electron microscopists, who later identified theit function
• Clathrin-mediated endocytosis occurs continuously throughout the cell

Cargo Selection by Adaptor Proteins

• Adaptor proteins bind cargo to initiate coat formation, leading to vesicle formation
• Clathrin coats detach soon after budding

Protein Retrieval

• Vesicles formed via clathrin-mediated endocytosis can either be broken down in lysosomes or recycled back to the surface
• Specific proteins are retrieved from early endosomes and sent back to the plasma membrane

Endocytosis Hotspots

• The back of a migrating cell is often a hotspot for endocytosis

Pinocytic Vesicles

• Not all pinocytic vesicles are clathrin-coated
• Cells "drink" the media around them by using membrane ruffles to form macropinosomes

Rab Proteins

• Rab proteins attach to vesicles and help them fuse with the correct target membranes
• Early endosomes-Rab5

Rab Proteins and Vesicle Transport

• Rab proteins are crucial for directing transport vesicles to target membranes
• Rab proteins are activated by GTP binding, similar to Rac and Rho
• Once activated, they can bind downstream effectors on membranes, triggering fusion

ER to Golgi

• COPII coats recruit cargo from the ER lumen and transport it to the Golgi lumen

Sorting Signals and Retrieval

• KDEL receptors and COPI coats ensure that any escaped ER resident proteins are returned
• Retrieval pathway to the ER uses sorting signals

Golgi Apparatus Compartments

• The Golgi apparatus is an ordered series of compartments
• Including cis Golgi network (CGN), cis cisterna, medial cisterna, trans cisterna, and trans Golgi network (TGN)

Golgi Transport

• Transport through the Golgi apparatus can happen through cisternal maturation
• Vesicle transport mechanism: cisternae stay put and vesicles move cargo through the stacks
• Cisternal maturation mechanism: cisternae move, and vesicles maintain Golgi resident enzymes in the correct region

Endosomes

• Endosomes serve as an intermediate stop for molecules entering and exiting the cell

Rapid Traffic

• Rapid intracellular transport occurs during 2D cell migration

Types of Exocytosis

• Constitutive secretion in migrating cells occurs at the polarized leading edge
• Regulated secretion occurs at nerve cell synapses to transmit action potentials

Trafficking Summary

• Intracellular trafficking involves a regulated and directional process
• Endocytosis brings the outside into the cell and is important for metabolism/signaling
• Coat proteins help in cargo recruitment and vesicle formation
• Rab proteins guide vesicles to target membranes
• Secretory pathway: ER → Golgi → endosomes → plasma membrane
• This pathway is a critical process for cell migration and development