Lecture 12 - Intracellular Trafficking - BIO 211 PDF

BIO 211: Cell, Molecular and Developmental Biology II Drexel University, Winter 2025 Lecture #12 – Intracellular Trafficking  OVERVIEW GENERAL PRINCIPLES RAB PROTEINS ER TO GOLGI TRANSPORT THROUGH GOLGI ENDOCYTOSIS EXOCYTOSIS The cytoplasm is chaos Introduction...

BIO 211: Cell, Molecular and Developmental Biology II Drexel University, Winter 2025 Lecture #12 – Intracellular Trafficking  OVERVIEW GENERAL PRINCIPLES RAB PROTEINS ER TO GOLGI TRANSPORT THROUGH GOLGI ENDOCYTOSIS EXOCYTOSIS The cytoplasm is chaos Introduction This apparent chaos is a tightly regulated system of directional transport within the cell. This system moves membrane enclosed vesicles and their associated cargo between organelles and the plasma membrane at the cell surface. Much remains to be discovered… Cells can capture and release molecules at the plasma membrane Exocytosis releases molecules such as cytokines into the environment. Endocytosis can remove molecular signals from the extracellular environment. Topology is preserved within the trafficking system During endocytosis, the outside of the cell becomes the inside (lumen) of the vesicle. When the vesicle fuses with its target compartment, the inside (lumen) of the vesicle is released into the inside (lumen) of the organelle. There Are Various Types of Coated Vesicles Protein coats help vesicles package cargo and bud from their membrane of origin The Assembly of a Clathrin Coat Drives Vesicle Formation Clathrin coats form mainly at the inner surface of the plasma membrane during endocytosis. The Assembly of a Clathrin Coat Drives Vesicle Formation Clathrin coats were spotted first by electron microscopists before we knew their function. Clathrin mediated endocytosis is happening all the time, all over the cell GFP-clathrin heavy chain Adaptor Proteins Select Cargo into Clathrin-Coated Vesicles Adaptor proteins connect cargo to coat formation and lead to vesicle formation. The clathrin coats fall off soon after budding. Specific Proteins Are Retrieved from Early Endosomes and Returned to the Plasma Membrane The cargo transported by vesicles formed by clathrin-mediated endocytosis can be destroyed in lysosomes and/or recycled back to the surface. The back of a migrating cell is a hot-spot for endocytosis Not All Pinocytic Vesicles Are Clathrin-Coated Cells can “drink” the media that surrounds them by using membrane ruffles to form macropinosomes. Cells can “drink” the media that surrounds them by using membrane ruffles to form macropinosomes. Phosphatidyl- serine Rab proteins bind to the outside of vesicles and help them fuse with their correct target membranes. Early endosomes – Rab5 Rab Proteins Guide Transport Vesicles to Their Target Membrane Rab Proteins Guide Transport Vesicles to Their Target Membrane Rab proteins are activated by GTP binding (similar to Rac and Rho). Once active, they are able to bind to their downstream effectors on target membranes to trigger fusion. ER to Golgi trafficking Proteins Leave the ER in COPII-Coated Transport Vesicles COPII coats recruit cargo from the lumen of the ER and move it to the lumen of the Golgi. The Retrieval Pathway to the ER Uses Sorting Signals KDEL receptors and COPI coats make sure that any ER resident proteins that escape are brought back to the where they are supposed to be. The Golgi Apparatus Consists of an Ordered Series of Compartments Transport Through the Golgi Apparatus May Occur by Cisternal Maturation The cisternae move and vesicles are The cisternae stay put and vesicles critical to keep the Golgi resident move the cargo through the stacks. enzymes in the correct region of the organelle. Introduction Endosomes are an intermediate stop on the journey into and out of the cell Rapid intracellular transport during 2D cell migration GFP-VSVG (Golgi to plasma membrane) Two types of exocytosis Constitutive secretion occurs in migrating cells with a polarized leading edge Regulated secretion occurs at nerve cell synapses to transmit action potentials SUMMARY OF TRAFFICKING Seemingly chaotic random process that is highly regulated and directional Endocytosis brings the outside inside, important for metabolism and signaling. Coat proteins help recruit cargo and form vesicles Rab proteins help direct the vesicles to their target membranes The secretory pathway (ER à Golgi à endosomes à plasma membrane) Is a critical process for cell migration and development