Golgi complex

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Deals with structure and function of golgi complex

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GOLGI COMPLEX Mrs. Rani Ashok Assistant Professor in Zoology Lady Doak College, Madurai - 2 eaarani@rediffmail.com

Introduction : 

Introduction First studied by Golgi Processing in golgi could be traced by autoradiography Processes proteins produced by RER and sends to their destination.

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Ordered series of Golgi compartments - Cisternae, tubular connections

Functions of the Golgi Complex : 

Functions of the Golgi Complex 1) sort proteins and lipids received from the ER; 2) modify certain proteins and glycoproteins; and 3) sort and package these molecules into vesicles for transport to other parts of the cell or secretion from the cell. 4) modification of amino acids (e.g.proline -> hydroxyproline) 5) addition of fatty acids

Structure of Golgi: based on function and morphology : 

Structure of Golgi: based on function and morphology cis-Golgi network: network of tubular membranes closest to ER a) function = sorting proteins i) returns ER proteins to sender ii) forwards remainder to cis-Golgi cisternae

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a) subdivided into cis, medial, and trans-cisternae b) each performs specific functions involved in processing proteins, has specific enzymes i) many are involved in glycosylation ii) also modify some proteins a) remove portions b) modify amino acids, e.g. convert proline to hydroxyproline Golgi cisternae: flattened stacks of membranes

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a) function = sorting proteins, sending to final destination Now include ERGIC (Endoplasmic Reticulum- Golgi Intermediate compartment) between ER and Golgi, as region where RER is morphing into cis-Golgi network - tubules starting to form certain proteins are only found in this region eg. ERGIC- 53 Trans-Golgi network: network of tubular membranes farthest from ER

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Golgi complex finishes, sorts, and ships cell products Golgiapparatus “Receiving” side ofGolgi apparatus Transportvesiclefrom ER Newvesicleforming Transport vesiclefrom the Golgi Golgi apparatus “Shipping”side of Golgiapparatus

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Rough ER Transport vesicle(containing inactivehydrolytic enzymes) Golgiapparatus Plasmamembrane LYSOSOMES “Food” Engulfmentof particle Foodvacuole Digestion Lysosomeengulfingdamagedorganelle

Transport from RER to Golgi : 

Transport from RER to Golgi Proteins (& lipids) move from site of synthesis to tips of RER COPII-coated vesicles transport materials from tips of RER to cis-Golgi network via ERGIC

Glycosylation – for stabilization of protein : 

Glycosylation – for stabilization of protein 1) complex carbohydrates are assembled stepwise on dolichol phosphate (a lipid) by glycosyltransferases starts on cytoplasmic face, then flips to lumenal face once 2 NAG and 5 mannose are added transfer 4 more mannose from dolichol-P, then 3 glucose 2) complex carbohydrate is transferred from dolichol phosphate to asparagine on target protein 3) all proteins of the endomembrane system are modified in this way

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Endocytic and biosynthetic-secretory pathways Transport vesicles (Ten or more chemically distinct, membrane-enclosed compartments)

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The biosynthetic-secretory and endocytic pathways

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Various types of coated vesicles Golgi apparatus Plasma ER and Golgi Cisternae

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Assembly of a clathrin coat triskelion Coated pits and vesicles on the cytosolic surface of membranes 36 triskelions 12 pentagons 6 hexagons Inner layer binds adaptins

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Adaptin binds to cargo receptor and clathrin triskelion Dynamin pinches off the bud Four types of adaptins Auxillin-activated ATPase is required To remove the clathrin coat Vesicles can have different shapes

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Dynamin pinches of the vesicles GTPase Shibire mutant has coated pits but no budding off of synaptic vesicles

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ARF proteins: COPI&clathrin Sar1 protein: COPII GTP causes Sar1 to Bind to membrane Assembly and disassembly of coat by GTPases GTPase works like a timer And cause disassembly shortly After the budding is completed

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SNAREs: specificity and fusion Rab GTPases: initial docking and tethering of vesicles to target membranes and matching of v- and t- SNAREs Guidance of vesicular transport

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SNARE proteins guide vesicular transport 20 SNAREs, v-SNAREs, t-SNAREs

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SNAREs specify compartment identity and control specificity 4 a helices in trans-SNARE complexes

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Rab proteins ensure the specificity of vesicle docking >30 Rabs On cytosolic surface C-terminal regions are variable: Bind to other proteins, including GEFs

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SNARE complex After docking SNAREs may mediate membrane fusion

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Entry of enveloped viruses into cells HIV Similar to SNAREs

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Proteins leave the ER in COPII-coated transport vesicles ER exit sites (no ribosomes) Selective process

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Only properly folded and assembled proteins can leave the ER Chaperones cover up exit signals

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Homotypic membrane fusion to form vesicular tubular clusters

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Vesicular tubular clusters - Lacks many of the ER proteins - Short-lived COPI-coated Retrograde transport: carry back the ER resident proteins that “leaked” out

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ER retrieval signals: KKXX in ER membrane proteins, KDEL sequence in soluble ER resident proteins - pH controls affinity of KDEL receptors

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Thank You!