Bio110 Lecture 8 - Transport in the Cell PDF

Transport processes within cells Nuclear transport – getting into and out of the nucleus Used to move proteins in and out of the nucleus, and RNA out of the nucleus (mRNA, rRNA, tRNA) Transport of protein through the Endomembrane system Used to move non-cytosolic pro...

Transport processes within cells Nuclear transport – getting into and out of the nucleus Used to move proteins in and out of the nucleus, and RNA out of the nucleus (mRNA, rRNA, tRNA) Transport of protein through the Endomembrane system Used to move non-cytosolic proteins throughout the cell Vesicles are used to carry proteins to other organelles or to send them out of the cell Endocytosis – getting material into the cell in bulk (or bringing in large objects) Used to move things into the cell Used to send things to the lysosome for recycling Use of the cytoskeleton in transport and cell movement Vesicles are dragged along the cytoskeleton Can also be used (in specialized structures) to move the cell Nuclear Transport  The Nuclear Envelope The nuclear envelope has two membranes, each consisting of a lipid bilayer, and is continuous with the endoplasmic reticulum. The inside surface forms a lattice-like sheet that stiffens the membrane’s structure, maintains its shape and provides attachment points for each chromosome The envelope contains thousands of openings called nuclear pores. – Function as portals into and out of the nucleus Nuclear Pores Each nuclear pore is a complex of over 50 different proteins These proteins determine what gets in and out (not size – many small molecules are still kept out) Nuclear Pores: Getting In & Out of the Nucleus Messenger RNAs (mRNAs) and ribosomes are synthesized in the nucleus and exported to the cytoplasm. Materials such as proteins that are needed in the nucleus are imported from the cytosol. Movement of proteins and other large molecules into and out of the nucleus is an energy-demanding process that utilizes special protein “importers”. Proteins destined for the nucleus have a molecular “zip code”— a nuclear localization signal (NLS) — which allows them to enter the nucleus. Proteins leaving the nucleus have a nuclear export signal (NES) that allows them to leave The Endomembrane System The Endomembrane System  The Endomembrane System The endomembrane system is composed of the rough and smooth ER and the Golgi apparatus, and is the primary system for protein and lipid synthesis, respectively. Ions, ATP, amino acids, and other small molecules diffuse randomly throughout the cell, but the movement of most proteins and other large molecules is tightly regulated. Entering the ER: The Signal Hypothesis  The Signal Hypothesis The signal hypothesis predicted that proteins targeted to the endomembrane system have a “zip code” that directs the polypeptide to the ER. This “zip code” is an ER signal sequence. The ER signal sequence binds to a signal recognition particle (SRP) that then binds to a receptor in the ER membrane. The SRP is a complex of protein and RNA In the RER lumen, proteins are folded and glycosylated (carbohydrates are attached to the protein). Leaving the ER: One Secretory Pathway The Golgi Apparatus Sorts the Cargo  The Golgi: A “Postal Sorting Facility” The Golgi apparatus’s composition is dynamic. New cisternae form at the cis face. Old cisternae break off from the trans face. Proteins enter the Golgi at the cis face and pass through cisternae containing enzymes for attaching specific carbohydrate chains, before exiting on the trans face of the Golgi. Glycosylation in the Golgi further sorts the proteins into vesicles bound for their ultimate destination The Golgi: A “Postal Sorting Facility”  Leaving the Golgi Each protein that comes out of the Golgi apparatus has a molecular tag that places it in a particular type of transport vesicle. Each type of transport vesicle also has a tag that allows it to be transported to the correct destination. Proteins produced in a cell have distinctive molecular address labels, which allow proteins to be shipped to the specific compartments where they perform their function. Some proteins are sent to the cell surface in vesicles that fuse with the plasma membrane, emptying their contents into the extracellular space in a process called exocytosis. Bulk Transport of materials to Lysosomes  Delivery to the lysosome for degradation Materials are delivered to the lysosomes by three processes: Phagocytosis Autophagy Receptor-mediated endocytosis Endocytosis is a process by which the cell membrane can pinch off a vesicle to bring outside material into the cell. Once in the lysosome, macromolecules are hydrolyzed.  Delivery to the lysosome for degradation 1. Receptor-mediated Outside Cytosol endocytosis uses receptors to cell bind to macromolecules outside the cell. Plasma membrane pinches in to form a vesicle that Recycling of receptors delivers cargo to lysosome. Receptor-mediated endocytosis H+ H+ 3. Phagocytosis brings smaller Endocytic vesicle Early endosome Late endosome cell or food particle inside cell, Lysosome forming a phagosome, which is delivered to the lysosome and Phagocytosis digested. 5. Autophagy encloses a Phagosome damaged organelle within a membrane, forming an autophagosome that is Autophagy delivered to the lysosome and digested. Damaged organelle Autophagosome The Cytoskeleton: Moving down the roads Green = Microtubules Red = Actin filaments Blue = Nucleus  The Cytoskeleton is used for transport & movement The cytoskeleton, composed of protein fibers, gives the cell shape and structural stability, and facilitates cell movement and transport of materials within the cell. In essence, the cytoskeleton organizes all of the organelles and other cellular structures into a cohesive whole. Microtubles (tubulin) are like the interstates of the cell – they provide a pathway for delivering molecules Intermediate filaments provide structure & support Actin filaments shape the plasma membrane, as well as anchor and move cells Cytoskeletal Structures Protein: many (e.g. keratin, lamin) tubulin actin  Microtubule Structure Microtubules are large, hollow tubes made of tubulin dimers. They are directional (proximal “-” end, distal “+” end) Microtubules originate from the microtubule organizing center and grow outward, radiating throughout the cell. Animal cells have just one microtubule organizing center called the centrosome. Microtubules act as “highways” – transport vesicles are carried down microtubules by the protein kinesin Microtubules separate chromosomes during cell division The Microtubule Organizing Center Transport down Microtubule highways  Actin Filaments Actin filaments are the smallest cytoskeletal elements. formed by polymerization of individual actin proteins. Also directional (have a “-” end and “+” end) Actin filaments are grouped together into long bundles that are usually found just inside the plasma membrane and help define the cell’s shape. Actin filaments are involved in movement by interacting with the protein myosin. Actin-myosin interactions cause cell movements such as cell crawling, cytokinesis, and cytoplasmic streaming. Actin is responsible for muscle tissue contraction Actin-mediated movements  Moving Whole Cells: Cilia & Flagella Flagella are long, hair-like projections from the cell surface that move cells. Bacterial flagella are made of flagellin and rotate like a propeller. Eukaryotic flagella are made of microtubules and wave back and forth. Closely related to eukaryotic flagella are cilia, which are short, filament-like projections. In addition to moving unanchored cells, cilia move mucus, nutrients, etc. in our airways and digestive tracts Cells generally have just one or two flagella but may have many cilia. Moving Whole Cells: Cilia & Flagella The Axoneme: A complex machine powers the movement of cilia & flagella (a) Transmission electron micrograph of axoneme (c) Mechanism of axoneme bending Microtubule doublet Central microtubules + + Microtubule doublet Dynein arms “walk” 75 nm along one side of Link adjacent doublets (b) Structure of axoneme Dynein Central arms 9 1 microtubules – – Spoke 2 + ATP: Dynein “walks” to minus end and 8 Microtubule causes linked doublets to bend Plasma membrane doublet 3 7 Link 6 4 Dynein 5 arms – end