Membrane Trafficking - BIO 201 Lecture Notes PDF

MEMBRANE TRAFFICKING RADHIKA BHARDWAJ, PHD DEPARTMENT OF BIOTECHNOLOGY, AIU BIO 201 Outlines: THE ENDOPLASMIC RETICULUM The Endoplasmic Reticulum and Protein Secretion Targeting Proteins to the Endoplasmic Reticulum Insertio...

MEMBRANE TRAFFICKING RADHIKA BHARDWAJ, PHD DEPARTMENT OF BIOTECHNOLOGY, AIU BIO 201 Outlines: THE ENDOPLASMIC RETICULUM The Endoplasmic Reticulum and Protein Secretion Targeting Proteins to the Endoplasmic Reticulum Insertion of Proteins into the ER Membrane Export of Proteins and Lipids from the ER Outlines: THE GOLGI APPARATUS Organization of the Golgi Protein Glycosylation within the Golgi Lipid and Polysaccharide Metabolism in the Golgi Protein Sorting and Export from the Golgi Apparatus Outlines: THE MECHANISM OF VESICULAR TRANSPORT Cargo Selection, Coat Proteins, and Vesicle Budding Vesicle Fusion LYSOSOMES Lysosomal Acid Hydrolases Endocytosis and Lysosome Formation Phagocytosis and Autophagy  Membrane trafficking A key process for maintaining the sustainability of the cell by transporting the nutrients and other solutes to all parts of the cellular system.  Membrane trafficking The wide variety of processes that go into the movement of cargo (typically proteins, pathogens and other macromolecules) using membrane bound transport vesicles.  Membrane trafficking Like a parcel sorting office, the cell uses a complex, highly regulated system to make sure that the right cargo is delivered to the correct location  Membrane trafficking This transport can take place within different organelles in the same cell, or across the cell membrane to and from the extracellular environment. Membrane trafficking is divided into two basic movement pathways: Based on the direction of travel Endocytic Pathway / Secretory Pathway / Endocytosis Exocytosis Exocytosis refers to the movement of cargo to the plasma membrane or out of the cell. As part of the biosynthetic-secretory pathway, newly synthesized proteins, lipids or carbohydrates move from the endoplasmic reticulum (ER) via the Golgi to the cell membrane or extracellular space. Conversely, Endocytosis is the movement of cargo into the cell from the plasma membrane. This can be often used for the uptake of nutrients which cannot be synthesized by the cell, such as vitamins, cholesterol and iron.  Protein trafficking Once proteins are made, they must be sent to the correct location Transport vesicles mediate membrane trafficking  Mechanisms of protein trafficking Vesicle mediated trafficking Non-vesicle mediated trafficking Involves proteins made Involves the proteins made in in the ER ribosomes free or cytosolic ribosomes  Overview of protein sorting in higher eukaryotic cells Proteins synthesized on membrane-bound Proteins synthesized ribosomes are translocated on free ribosomes into the ER while their either remain in the translation is in progress. cytosol or are transported to the Either retained within the nucleus, mitochondria, ER or transported to the chloroplasts, or Golgi apparatus and, from peroxisomes. there, to lysosomes, the plasma membrane, or the cell exterior via secretory vesicles.  Secretory pathway Rough ER ~ Golgi ~ Secretory vesicles ~ Cell exterior The entrance of proteins into the ER thus represents a major branch point for the traffic of proteins within eukaryotic cells. Proteins destined for secretion or incorporation into the ER, Golgi apparatus, lysosomes, or plasma membrane are initially targeted to the ER. DISTRIBUTIONS OF COAT PROTEINS ALONG THE SECRETORY PATHWAY Clathrin, Coat protein I (COPI), Coat protein II (COPII) Coats are color coded ER export sites (ERES) Vesicular tubular carriers (VTCs) OVERVIEW OF THE SECRETORY MEMBRANE SYSTEM Transport carriers move lipids and proteins between the three principal organelles of the secretory pathway—the endoplasmic reticulum (ER), Golgi apparatus, and plasma membrane. The carriers are either small vesicles or larger vesicle–tubule elements that move along microtubules (brown lines) between membranes. Carriers form at ER export sites (ERES), producing vesicular tubular carriers (VTCs) that move to the Golgi apparatus. The endoplasmic reticulum, Golgi apparatus, endosomes, and lysosomes are thus distinguished from other cytoplasmic organelles by their common involvement in protein processing and connection by vesicular transport  OVERVIEW OF THE SECRETORY MEMBRANE SYSTEM The Anterograde Pathway The Retrograde Pathway Retrograde transport carriers bud Takes newly synthesized proteins, from the VTC or Golgi apparatus to called cargo, from the ER to the Golgi retrieve proteins and lipids back to apparatus, and from the Golgi the ER. apparatus to the plasma membrane or to the endosome/lysosomal system. A different set of retrograde carriers retrieve proteins and lipids from the endosomal/lysosomal system back to the trans-Golgi network (TGN). PROTEIN MACHINERY FOR SECRETORY TRANSPORT Proteins transported through the secretory system are called Cargo Coat proteins help sort soluble cargo and transmembrane proteins into a coated bud that pinches off a donor membrane as (A) a coated vesicle or (B) larger vesicular tubular carriers. PROTEIN MACHINERY FOR SECRETORY TRANSPORT Motor proteins - move carriers along either microtubules (shown here) or actin filaments. - Long coiled-coil tethers or multimeric tethering complexes attach carriers to an acceptor membrane. PROTEIN MACHINERY FOR SECRETORY TRANSPORT SNARE (soluble N ethylmaleimide-sensitive factor attachment protein receptor) SNARE proteins on the carrier and acceptor membrane then form a complex that drives membrane fusion and delivery of the carrier’s membrane and content to the acceptor membrane. During this process, the relative topology of the lipids and transmembrane proteins is maintained Vesicular transport from the ER to the Golgi Proteins and lipids are carried from the ER to the Golgi in transport vesicles that bud from the membrane of the transitional ER, fuse to form the vesicles and tubules of the ER- Golgi intermediate compartment (ERGIC) and are then carried to the Golgi. Lumenal ER proteins are taken up by the vesicles and released into the lumen of the Golgi. Membrane proteins maintain the same orientation in the Golgi as in the ER. The Golgi apparatus, or Golgi complex Retrieval of resident ER proteins Proteins destined to remain in the lumen of the ER are marked by the sequence Lys-Asp-GluLeu (KDEL) at their carboxy terminus. These proteins are exported from the ER to the Golgi, but they are recognized by a receptor in the ERGIC or the Golgi apparatus and selectively returned to the ER  Organization of the Golgi The Golgi apparatus is composed of flattened membrane-enclosed sacs that receive proteins from the ER, process them, and sort them to their eventual destinations.  Regions of the Golgi apparatus Vesicles from the ER fuse to form the ER-Golgi intermediate compartment, and proteins from the ER are then transported to the cis Golgi network. Resident ER proteins are returned from the ER- Golgi intermediate compartment and the cis Golgi network via the recycling pathway. The medial and trans compartments of the Golgi stack correspond to the cisternae in the middle of the Golgi complex and are the sites of most protein modifications. Proteins are then carried to the trans Golgi network Transport from the Golgi apparatus Proteins are sorted in the trans Golgi network and transported in vesicles to their final destinations. In the absence of specific targeting signals, proteins are carried to the plasma membrane by constitutive secretion. Alternatively, proteins can be diverted from the constitutive secretion pathway and targeted to other destinations, such as endosomes and lysosomes or regulated secretion from the cells. The Mechanism of Vesicular Transport Cargo Selection, Coat Proteins, and Vesicle Budding Most transport vesicles that carry secretory proteins from the ER to the Golgi and from the Golgi to other targets are coated with cytosolic coat proteins and thus are called coated vesicles Three kinds of coated vesicles have been characterized: COP-coated vesicles, COPI and COPII (COP indicates coat protein), and clathrin-coated vesicles. Formation and fusion of a transport vesicle Membrane and lumenal secretory proteins are collected into selected regions of a donor membrane where the formation of a cytosolic coat results in the budding off of a transport vesicle. During transport the coat is disassembled, and the transport vesicle docks and fuses with the target membrane. Vesicle Fusion  Vesicle Fusion The fusion of a transport vesicle with its target involves two types of events: First, the transport vesicle must recognize the correct target membrane; for example, a vesicle carrying lysosomal enzymes has to deliver its cargo only to lysosomes. Second, the vesicle and target membranes must fuse, delivering the contents of the vesicle to the target organelle. Protein Trafficking, I-Cell Disease, Clathrin, Vesicular Transport & Protein Modifications - YouTube