Protein trafficking in lysosomes

Protein trafficking in lysosomes

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  PROTEIN TRAFFICKING IN LYSOSOMESAND VESICULAT TRANSPORT NAME: Sheryl Bhatnagar Roll no.: 2047 Course: Botany(h) 3rd yr
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  Contents • Introduction • Translocationpathways • Lysosomes • Modifications of lysosomal proteins • Vesicular transport • Coated vesicles • Clathrin coated vesicles • Formation of coated vesicles • Vesicle fusion
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  Protein trafficking • Proteintrafficking is the mechanism by which a cell transports proteins to the appropriate positions in the cell or outside of it. • Both in prokaryotes and eukaryotes, newly synthesized proteins must be delivered to a specific subcellular location or exported from the cell for correct activity. This phenomenon is called protein trafficking. • This delivery process is carried out based on information contained in the protein itself. • Correct trafficking is crucial for the cell; errors can lead to diseases. • In 1970, Günter Blobel conducted experiments on the translocation of proteins across membranes. • He was awarded the 1999 Nobel Prize for his findings. He discovered that many proteins have a signal sequence, that is, a short amino acid sequence at one end that functions like a postal code for the target organelle.
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  Translocation Pathways CO- TRANSLATIONALPATHWAY POST-TRANSLATIONAL PATHWAY • Transport occurs while the polypeptide chain is being synthesized on a membrane-bound ribosome. • Signal sequences are bound by signal recognition particle (SRP) . • The polypeptide chain is completed in the cytoplasm before being transported into the endoplasmic reticulum. • Signal sequences recognized by receptors on translocon (not need SRP) .
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  Lysosomes • Lysosomes arespherical membranous bags that contain enzymes (Acid Hydrolases). • size varies from 0.1–1.2 μm. • The lysosomal enzyemes are capable of digesting all varieties of biological molecules  Functions: I. Digestion of worn out or non functional organelles. II. Metabolic functions III. Degradation of nonuseful tissue
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  Some important enzymesfound within lysosomes include:
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  Modifications of LysosomalProteins • The processing of N-linked oligosaccharides of lysosomal proteins differs from that of plasma membrane and secreted proteins. • The lysosomal proteins are modified by mannose phosphorylation. • First, there is addition of N-acetylglucosamine phosphates to specific mannose residues, and this happens probably while the protein is still in the cis Golgi network. • After this N-acetylglucosamine group is removed, leaving mannose-6-phosphate residues on the N-linked oligosaccharide. • The phosphorylated mannose residues are specifically recognised by a mannose-6- phosphate receptor in the trans Golgi network, which directs the transport of these proteins to lysosomes.
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  Vesicular transport • Proteinsfrom the ER to the Golgi apparatus and proteins to E.R and from golgi to cell organelles, for example, occurs in this way. • transport intermediates— which may be small, spherical transport vesicles or larger, irregularly shaped organelle fragments—carry proteins from one compartment to another.
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  Coated vesicles Role ofthe coat:  Components of the membrane (e.g. receptors) are concentrated into patches  Removal of coated surfaces and formation of vesicles Types of coated vesicles:  Clathrin-coated vesicles  COPI-coated vesicles  COPII-coated vesicles Clathrin-coated vesicle: Golgi – surface mamembrane transport COPI- and COPII-coated vesicles: rER - Golgi transports
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  Clathrin-coat Triskelion zikk-zakk andglobular structural elements
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  The Formation ofcoated vesicles • The formation of coated vesicles is regulated by small GTP-binding proteins related to Ras and Ran. • Two families of GTP-binding proteins play roles in transport vesicle budding: ADP-ribosylation factors (ARFs 1-3 & Sarl) and a large family of Rab proteins. • These regulate adaptor proteins that interact directly with a vesicle coat protein. • The binding of GTP-binding proteins and adaptor proteins establishes a "platform" on the membrane for a specific process, such as assembly and budding of a transport vesicle directed from the transitional ER to the Golgi or from the trans Golgi network to endosomes and lysosomes.
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  •Individual proteins inthe complex (coat proteins, adaptor proteins, and GTP-binding proteins) may participate in assembly of transport vesicles directed elsewhere, or in vesicle,but each protein complex is apparently unique to a particular budding, transport, or fusion pathway.
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  Vesicle Fusion • Thefusion 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, deliver- ing the contents of the vesicle to the target organelle. • Analysis of the proteins involved in vesicle fusion in these systems led Rothman and his colleagues to pro- pose a general model, called the SNARE hypothesis, in which vesicle fusion is mediated by interactions between specific pairs of transmembrane proteins, called SNAREs, on the vesicle and target membranes (v-SNAREs and t-SNAREs, respectively). • .
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  According to thehypothesis, the formation of complexes between v- SNAREs on the vesicle and t-SNAREs on the target membranes leads to membrane fusion
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  Cont. • SNAREs arerequired for vesicle fusion with a target membrane and that SNARE-SNARE pairing provides the energy to bring the two bilayers sufficiently close to destabilize them and result in fusion. • However, the docking, tethering, and fusion of transport vesicles to specific target membranes appears to be mediated by a sequentially assembled protein complex much like that which led to transport vesicle budding. • Members of the Rab family of small GTP-binding proteins play key roles in this docking of transport vesicles. • Rab proteins, like the ARF family, participate in many of the vesicle budding and fusion reactions during vesicular transport. • More than 60 different Rab proteins have been identified and shown to function in specific vesicle transport processes. • They function in many steps of vesicle trafficking, including interacting with SNAREs to regulate and facilitate the formation of SNARE/SNARE complexes.