Endoplasmic Reticulum and Golgi Apparatus

Endoplasmic Reticulum – "mother of all membranes"

ER is a labyrinth of membranes where all membrane proteins are made – enclosed space is topologically outside of cell

rough ER has ribosomes and can do protein synthesis; smooth has no ribosomes

Five Functions of ER:

(1) Protein Synthesis (rough ER only)

mRNA synthesized in nucleus is exported to cytoplasm via pores and attaches to cytoplasmic ribosome
some proteins are released into cytoplasm; others have targeting signals that send them to ER

signal sequence – 15-20 amino acids on N terminus – positive then hydrophobic residues

many different sequences – each highly specific for targetting location
recognized by SRP (signal recognition particle) early in translation, attaches to ER at specific point
protein is fed through ER – hairpin turn inside ER produces 2 transmembrane regions
when translation is complete: one end outside, other inside – cleaved for secretion via signal peptidase
this is how multiple transmembrane proteins (e.g., CFTR)formed; hydrophobic regions stay in membrane

cotranslational modification – occur during translation while protein is rushing across membrane of ER

formation of disulfide bonds (cysteine), action of signal peptidases, and formation of N-glycans (carbohydrates added to Asn)

conformational maturation (folding) – produces bonds that form tertiary and quaternary protein structure

deficiency produces proteins that are not secreted appropriately
e.g., anti-trypsin deficiency (buildup in liver), familial hypercholesterolemia (LDL receptor not to surface)

(2) Phospholipid Synthesis

(3) Glycogen Metabolism

(4) Drug Detoxification by Cytochrome P-450

(5) Import/Export by specialized routes to and from the cytoplasm

Golgi Apparatus

The Golgi apparatus was discovered in 1898 by mistake (using silver stain on neurons) – silver blackens Golgi cis cisterna

The purpose of the Golgi is to prepare secretory proteins for secretion

proteins made in the ER form transitional element then pinch off in a vesicle

vesicle fuses with Golgi apparatus and migrates through Golgi cisternae (saucerlike – cis Þ medial Þ trans)

cis identified by silver stain, trans identified by thiamine phosphatase

trans Golgi network (TGN) – many decisions on secretory path made here

if constitutive, vesicle will then fuse with cell membrane at specific locations (exocytosis is specialized)
if regulated (neurons, exocrine pancreas, saliva, blood cells), accumulate near secretion site (secretory granules)
digestive proteins are shuttled to lysosomes – Golgi can direct secretions (e.g., hemagglutinin to basolateral)

some functions: (1) secretion, (2) concentration (even to crystallization!), (3) sorting, (4) proteolysis (e.g., of proinsulin)

Glycoproteins – biosynthesis occurs in the Golgi

There are three types of glycoprotein carbohydrate adducts, which can all exist on the same protein:

(1) N-glycans – oligosaccharide added to asparagine residue (asn-x-ser/thr) of nascent protein in ER

few chains – total only 5-20% by weight– nearly every membrane protein has N-glycans (e.g.,membrane Ig’s)

initial adduct linked to lipid – dolichol-P-P-(N-acetyl Glucose)₂-(Mannose)₉-(Glucose)₃

all three glucoses and one mannose residue are removed and sent to Golgi, and the oligosaccharide is extensively reworked Þ branches added

one example: (N-acetyl glucose)-(Galactose)-(Sialic Acid) added to terminal mannose

permeases allow sugars (e.g., UDP-galactose) to enter the Golgi for this purpose

purpose: binding/recognition–fertilization (egg+sperm), lymphocyte recognition of antigens, cholera toxin receptor

I cell disease – mannose-6-phosphate is added to terminal mannose to target protein to lysosome

if no mannose-6-phosphate added: digestive enzymes secreted – lysosomes cannot digest – grow huge
can correct if enzyme added exogenously (Hunter and Hurler cells each missing one enzyme; grow when plated together)

(2) O-glycans – very many added to each protein, so much so that carbohydrate usually outweighs protein (50-90% total)

added to serine or threonine one residue at a time – starts with N-acetyl galactose
often charged, so radical effects on protein structure – often make gels (interaction with water)

mucin – many O-glycans increase surface area to trap pathogens and protect cells

(3) Glycosaminoglycans (proteoglycans) – common in connective tissue (somewhat compressable)

very long chains; >90% total weight – usually highly negatively charged
added to serine or threonine – starts with (xylose)-(galactose)₂-(glucuronic acid)-(repeating disaccharide)
only use of xylose in humans

Endocytosis – uptake of soluble particles – most significant type is receptor-mediated

(1) LDL Receptor – internalized, pH of 5 causes release of LDL to lysosome (after signaling cell to stop making cholesterol), receptor recycled

(2) Transferrin Receptor – internalized, iron removed due to ß pH, receptor recycled and transferrin (binds iron at neutral pH) released outside the cell

(3) Epidermal Growth Factor Receptor – internalized, both receptor and EGF destroyed – suicidal mechanism

(4) Transcytosis of IgA – internalized, through cell (avoid lysosome and don’t return to basal surface), then release on apical surface – proteases destroy receptor on way and release protein