Intracellular Vesicular Traffic ER to Golgi Pages 749-766

Golgi Apparatus Functions include: Carbohydrate synthesis -pectin and hemicellulose in plants -extracellular matrix in animals Carbohydrate synthesis onto proteins Sorting of products from the ER

Types of Coated Vesicles ER exit sites

Formation of ER Transport Vesicles Proteins that need to return to the ER: Cargo receptors -Contain exit signals -Some are lectins (ERGIC53 – clotting) 2. Resident ER proteins

Antibody Retention/Secretion -Chaperone proteins (calnexin, BiP) retain misfolded or incomplete proteins in the ER -They cover up exit signals -ER imports peptide fragments -Cystic Fibrosis, misfolded protein is retained in ER

Homotypic Membrane -Fusion of membranes from the same compartment Vesicular tubular cluster -Heterotypic fusion – a membrane from one compartment fuses with the membrane of a different compartment

Vesicular Tubular Clusters -The clusters are short lived -They move along microtubules to the Golgi while some are involved with transport back to the ER

Retrieval of ER Resident Proteins -ER Retrieval signal -KKXX or KDEL -Lys-Asp-Glu-Leu -KDEL receptors can bind directly to COP I coat proteins

Retrieval of ER Resident Proteins -The affinity of the KDEL receptor changes depending on the compartment it is in -Some proteins do not have return signals and are returned to the ER at a slower rate

Kin Recognition -Retention of proteins within a compartment through their aggregation -ER resident proteins may aggregate forming complexes -Large number of proteins interacting with lower affinity

Golgi Apparatus Animal Cells

Golgi Apparatus Plant Cells

Golgi Apparatus ER side During passage through the Golgi, molecules are covalently modified

Golgi Compartmentalization Unstained Stained Acid Phosphatase trans Golgi network Nucleoside Diphosphatase trans

Golgi Compartmentalization -Oligosaccharide processing -Resident proteins of the Golgi are all membrane bound -Glycosidases and glycosyl transferases

Goblet Cell of Small Intestine -There are more Golgi in cells specializing in secretion

N-Linked Glycosylation -Glycoproteins – proteins containing sugar residues -Linked to Asparagines in the sequences Asn-X-Ser and Asn-X-Thr, where X cannot be Pro -Transfer is catalyzed by oligosaccharide transferase which has its active site on the lumenal side of the ER

Precursor Oligosaccharide Synthesis -The sugars are first activated in the cytosol by forming nucleotide-sugar intermediates -O-linked oligosaccharides are linked to hydroxyl groups of Ser and Thr residues and this occurs in the Golgi

Protein Glycosylation The precursor oligosaccharide is held in the ER membrane by a long and very hydrophobic lipid molecule, dolichol

Two Classes of Oligosaccharides

Two Classes of Oligosaccharides Most are complex -Trisaccharide unit – GlcNAc-Gal-Sialic acid

Oligosaccharide Processing

Models of Golgi Organization Static model Dynamic Model