Lecture 4

• Proteins received by the Golgi are further modified and sorted for transport to various cellular compartments including the plasma membrane
• Golgi receives newly synthesized and correctly assembled secretory cargo proteins from the ER
• The Golgi Apparatus is also known as golgi network, golgi complex, golgi body, golgi ribbon, dictyosome (in plants) or simply the 'Golgi'
• Golgi Machinery

  GRASPS, tethers
• In lower eukaryotes like yeast, golgi morphology is less organized with dispersed "mini-stacks" in the cytoplasm instead of a ribbon
• The Golgi Apparatus is a basket or ribbon-like organelle located at the perinuclear region
• Golgi Organization in vertebrate cells is characterized by parallel stacks of flattened membrane disks (cisternae) connected to form ribbons
• Some prokaryotes lack a discernible golgi altogether
• Until the 1950s, skeptics derided the discovery of the Golgi Apparatus as an artifact of Golgi's staining method
• The Golgi Apparatus is a polarized organelle with CGN receiving vesicles from the ER and TGN packaging secretory cargo for release
• Although morphologically identical, each compartment/cisterna in the Golgi is physically and functionally distinct
• The Golgi Apparatus was initially discovered in nerve cells by Camillo Golgi in 1898 using a silver staining method
• The Golgi Apparatus is distinct from nuclear, ER, and plasma membranes
• The Golgi Apparatus is further flanked on two sides by fenestrated tubular networks: the cis-golgi network (CGN) and trans golgi network (TGN)
• Parallel cisternae in the Golgi Apparatus are distinct from one another and organized into cis, medial, and trans compartments
• The Golgi Apparatus is organized around the centrosome/Microtubule Organizing Centre (MTOC)
• Golgi is temporarily fragmented into mini-stacks and individual cisternae during cell division
• Golgi Transport

  ER to the golgi
• Golgi Machinery

  Focus on golgi ribbon formation
• Tethering proteins (golgins) draw mini-stacks into close proximity to allow membrane fusion
• Golgi is functionally inactive during cell division
• Intact microtubule network is required to maintain ribbon structure and perinuclear localization
• Anterograde transport


  Newly assembled proteins move from ER to cis-Golgi in transport vesicles
• Golgi membrane fusion likely occurs by SNARE mediated docking mechanism as with vesicle membrane fusion
• Microtubule network and microtubule dependent transport clusters golgi mini-stacks at the perinuclear region
• Glycosylation in Golgi

  Many secretory cargo proteins are glycoproteins. Modified with sugar polymers or oligosaccharides/glycans (glycosylated). Initial transfer occurs in ER but glycans are remodified in the golgi. Glycosidases remove and glycosyltransferases add individual sugars to the chain
• Sorting cargo for transport

  Sorting signal on cytoplasmic domain of membrane cargo proteins are recognized by coat proteins and selected for inclusion in budding vesicle. Soluble cargo proteins with luminal sorting signals require recognition by membrane cargo receptors for transport selection. Proteins may also be randomly captured by budding vesicles
• GRASPs

  • Golgi Reassembly and Stacking Proteins. Membrane associated proteins that dimerize/oligomerize. (stacking protein to hold parallel cisternae together)
• Retrograde retrieval

  Sorting signals are involved in maintaining residency within a compartment. ER-resident proteins can be randomly moved to cis-golgi and need to be retrieved by active retrograde transport (COPI vesicles). Sorting signal – KDEL sequence. Slight difference in pH in ER versus cis Golgi determines binding of KDEL to a membrane receptor for retrieval
• During mitosis, reorganization of microtubules into spindles is accompanied by restructuring of the golgi
• Golgins


  • Coiled coil protein with extended rod-like conformation (tethering). Helical coiled coil structure
• Retrograde transport

  COPII coated vesicles move from ER to golgi (anterograde transport). COPI vesicles move in reverse direction (retrograde transport)
• Diseases associated with golgi: CDG (Congenital Defects of Glycosylation)
• Reflects importance of glycosylation on extracellular matrix proteins (e.g., collagen, elastin) and antibody-antigen recognition
• Golgi Function: Glycosylation in Golgi
• End Lecture 4: Transport through golgi apparatus results in modification and repackaging of proteins for targeting to final destination
• If secretory cargo remains stationary within the compartment how are they progressively modified (glycosylated) as cisternae matures?
• Clinical presentations of CDG

  • Psychomotor retardation
  • Cutis laxa (wrinkled inelasticitic skin)
  • Dysmorphic features
  • Autoimmune disease
  • Muscular dystrophy
  • Ataxia
• Didn’t you also say that each compartment had distinct set of sugar modifying enzymes?
• Golgi Function: Cisternal maturation