Cellular compartments 2 Endo and exocytosis LO: how material is sent out of a cell (secretion) Changes that may occur to a proteins structure after translation How we get material into a cell Secretion overview · Synthesis/ER-> Golgi-> Plas Memb/extracellular space (b) THE SECRETORY PATHWAY: A MODEL RNA Rough ER cis face of Golgi apparatus Golgi apparatus 1. Secreted proteins enter ER as they are being synthesized by ribosome. 2. Protein exits ER in vesicle. 3. Protein travels through the cisternae of the Golgi apparatus. 4. Protein enters a secretory vesicle that fuses with cell membrane. trans face of Golgi apparatus Plasma membrane 5. Protein is secreted from cell. Secretory pathway STEP 1: · Co-translational trans- membrane transport (most proteins start in the cytosol, exceptions are mitochondrial proteins, which are synthesised by mitochondrial DNA) most proteins are covalently modified in the ER common pool of ribosomal subunits in cytosol 5' ER signal sequence 5 3' 3 mRNA encoding a protein targeted to ER remains ER membrane membrane-bound polyribosome bound to ER membrane by multiple nascent polypeptide chains · Important changes to protein structure · Glycosylation o Sugars added to proteins (glycoproteins) o Major function of ER o Most soluble and membrane bound proteins are glycoproteins o Cytosolic proteins are simply or not glycosylated o Often essential for function Glycosylation in ER · A complex of 14 sugars is added en bloc · Sugars transferred from a lipid (dolichol) o Oligosaccharide protein transferase is able to bind to the sugars and scan incoming polypeptide chain. o Does this by looking out for Asn residues · To asparagine residue o Not all ASN are glycosylated NH2 NH2 CYTOSOL dolichol dolichol P Asn P P ER LUMEN P growing polypeptide chain Asn oligosaccharide protein transferase lipid-linked oligosaccharide o Not any Asn will do, must come in special consenus sequence o Conse nsus Asn-X-Ser/Thr · X=any amino acid (apart from proline) o N-linked glycosylation (Asn=N) ER chaperones ensure quality · Chaperones bind unfolded proteins · Binding keeps them in the ER · Important for correct folding and assembly of multimeric proteins · Antibodies o 4 polypeptide chains · BiP (chaperone) retains Ab in the ER until it is complete · Improperly folded proteins are exported and degraded in the cytosol
. Misfolded proteins in the ER activate Unfolded Protein Response (UPR) (stops translation of these proteins +increase chaperone proteins) Secretory pathway STEP 2 · Vesicular transport to the Golgi and onwards Glycosylation in the Golgi · O-linked glycosylation CYTOSOL LUMEN DONOR COMPARTMENT FUSION TARGET COMPARTMENT BUDDING · Poorly understood · Sugars bind to -OH groups of amino acid side chains (Ser/thr) · Less frequent than N-linked Exocytosis STEP 3 Secretion to the PM/extracellular space · Proteins are sorted in the trans Golgi network and transported in vesicles to their final destinations · Secretion o Constitutive o Regulated vesicle which has its newly synthesized soluble proteins for constitutive secretion newly synthesized plasma membrane lipids transport vesicle CONSTITUTIVE SECRETION unregulated membrane fusion newly synthesized plasma membrane