Cell Biology-Cellular Basis of Protein Synthesis and Secretions I and II PDF

Protein Synthesis & Secretion [1] Paul J. McDermott, Ph.D. [email protected] CELLULAR BASIS OF PROTEIN SYNTHESIS & SECRETION A. OVERVIEW OF PROTEIN SYNTHESIS, SORTING & SECRETION 1. Free Ribosomes 2. Membrane-bound Ribosomes B. ENDOPLASMIC RETICULUM 1. Structure 2. Types of ER 3. Continuity with Nucl...

Protein Synthesis & Secretion [1] Paul J. McDermott, Ph.D. [email protected] CELLULAR BASIS OF PROTEIN SYNTHESIS & SECRETION A. OVERVIEW OF PROTEIN SYNTHESIS, SORTING & SECRETION 1. Free Ribosomes 2. Membrane-bound Ribosomes B. ENDOPLASMIC RETICULUM 1. Structure 2. Types of ER 3. Continuity with Nuclear Envelope C. ROUGH ENDOPLASMIC RETICULUM (RER) 1. Synthesis of Secretory Proteins: Secretory Pathway 2. Transmembrane Proteins 3. Synthesis of Single-pass Transmembrane Proteins 4. Synthesis of Multi-pass Transmembrane Proteins 5. Protein Modifications in the RER 6. Protein Folding in the RER 7. Protein Quality Control in the RER D. SMOOTH ENDOPLASMIC RETICULUM (SER) 1. Structure 2. Main Functions of SER 3. Synthesis of Lipids E. GOLGI COMPLEX 1. Structure of the Golgi Complex 2. Compartments of the Golgi Complex 3. Tubular Vesicular Network or Compartment 4. Protein Processing in the Golgi Complex 5. Synthesis of Sphingomyelin and Glycolipids 6. Secretory Pathways of trans Golgi Network: 3 Main Types F. VESICULAR TRAFFICKING 1. Anterograde and Retrograde Transport in the Golgi Complex 2. Coated Transport Vesicles: 3 Established Types 3 Protein Sorting into Transport Vesicles 4. Sorting Sequences for Specific Types of Transport Vesicles 5. Budding of Transport Vesicles 6. Fusion of Transport Vesicles to Target Membranes 7. Orientation of Membrane and Secreted Proteins During Cellular Transport 8. Secretory Granules and Exocytosis Suggested Reading The Cell: A Molecular Approach, 2nd Ed. Ch. 9: http://www.ncbi.nlm.nih.gov/books/NBK9839 Junqueira’s Basic Histology Text and Atlas, 16e, © 2021, McGraw-Hill, Chapter 2 I look into the finance box Just to check my status I look into the microscope See Golgi Apparatus Golgi, oh, woe is me You can't even see the sea Golgi, olgi, oh ooo olgi Golgi Golgi Phish Protein Synthesis & Secretion [2] OBJECTIVES 1. Describe the structure of the ER and the differences between RER and SER. 2. Describe how secretory proteins are produced starting with mRNA and ending in the RER lumen. 3. Describe how an integral membrane protein is produced starting with mRNA and ending with a transmembrane protein inserted in a membrane. 4. Describe the functions of internal signal sequences (ISS) and stop transfer sequences (STS) in producing multi-pass transmembrane proteins. 5. Specify the types of protein modifications produced by RER and where in RER they occur. 6. Describe the functions of chaperones and Protein Disulfide Isomerase (PDI) in the RER. 7. Specify the main lipids that are synthesized by the SER and explain the role of flippase. 8. Identify the following components of the Golgi complex: cis Golgi network, medial and trans Golgi stacks, trans Golgi network, transport (shuttle) vesicles. 9. Specify 4 types of protein processing in the Golgi complex and pinpoint where they occur. 10. Specify 2 types of lipids synthesized in the Golgi complex. 11. Describe the basic schema of anterograde and retrograde transport of vesicles between RER, tubular vesicular network (TVN) and the Golgi complex. 12. Specify the 3 main types of coated vesicles and their respective roles in protein sorting and transport. 13. Describe how cargo proteins and transmembrane proteins are sorted into vesicles for transport. 14. Differentiate between constitutive and regulated secretion. 15. Describe the mechanisms underlying vesicular budding and fusion with target membranes. 16. Describe the function of sorting sequences and know the specific types of proteins that are sorted by Mannose-6-P. Illustrations and images adapted from: • The Cell: A Molecular Approach © 2000 ASM Press and Sinauer Associates, Inc. • Medical Cell Biology, 3rd Ed. © 2008, Elsevier Inc. • Molecular Cell Biology, © 2000 by W.H. Freeman and Company • Molecular Biology of the Cell © 2002, Garland Science Protein Synthesis & Secretion [3] A. OVERVIEW OF PROTEIN SYNTHESIS, SORTING & SECRETION Proteins are translated on ribosomes and must be either sorted and transported to the correct cellular location or directed to the secretory pathway. 1. “Free” Ribosomes: Translation occurs entirely in the cytoplasm. The nascent polypeptides contain sequences for targeting to the correct organelle or other cytosolic location. 2. Membrane-bound Ribosomes: A targeting sequence near the N-terminus of nascent polypeptides is used to direct ribosomes to the outer surface of the rough endoplasmic Fate of reticulum (RER). Upon completion of translation, the newly synthesized proteins are either protien afterincorporated into membranes or packaged for secretion. translation(2) • Translation always begins on free ribosomes in the cytosol. • Proteins destined for either extracellular secretion or lysosomes move through the RER and Golgi for processing, sorting and packaging into vesicles. Plasma membrane Mature secretory granules Cell exterior Coated secretory vesicles (4) Endosome B. ENDOPLASMIC RETICULUM (ER) 1. Structure • Network of membrane enclosed tubules • Largest membrane system in mammalian cells • Lumens of SER and RER are continuous • ER is continuous with outer membrane of the nuclear envelope RER Fig Lysosome SER cytoplasm lumen cytoplasm ER lumen 2. Types of ER a) Rough ER (RER) - Bound ribosomes active in translation b) Smooth ER (SER) - No bound ribosomes membrane membrane Protein Synthesis & Secretion [4] B. ENDOPLASMIC RETICULUM (ER) 3. Continuity with Nuclear Envelope The outer membrane of the nuclear envelope is continuous with ER membranes and the perinuclear space is continuous with the ER lumen. nuclear pore perinuclear space nuclear lamina Fig inner and outer nuclear membrane nuclear matrix chromatin RER • TEM of exocrine cell in the pancreas The cytoplasm contains an extensive amount of RER that is involved in protein synthesis and secretion. Note the ribosomes that are attached to the outer membrane of the nuclear RER fxn(2) envelope. euchromatin heterochromatin Fig RER lumen RER membrane Fig • High magnification TEM of RER Attached (membrane bound) and free ribosomes are visible in the cytoplasm. Protein synthesis is initiated on free ribosomes in the cytoplasm. Secretory and integral membrane proteins have an AA targeting sequence that directs the translating ribosome to the RER membrane. Nuclear Envelope inner membrane outer membrane Protein Synthesis & Secretion [5] C. ROUGH ENDOPLASMIC RETICULUM (RER) 1. Synthesis of Secretory Proteins: Secretory Pathway Translating Ribosomes Pathway(6) mRNA Golgi Complex RER Secretory Vesicles Exocytosis Step 1 3´ Signal Sequence or Peptide mRNA 5´ N mRNA 3´ 5´ CYTOSOL SRP Step 2 GDP•Pi Schematic pathway Step 4 Step 4 5´ Step 3 3´ Translocon 5´ 3´ 5´ 3´ RER MEMBRANE N SRP receptor RER LUMEN N Secretory protein Step 5 C N Signal peptidase Secretory pathway(5) Step 1: Signal Sequence (Signal Peptide) is synthesized as part of the first 16-30 amino acids SS fxn in the amino terminus of a secretory protein as it emerges from the ribosome during translation in the cytoplasm. It contains a hydrophobic domain that serves as a binding site for SRP. N Signal Sequence Secretory protein C 16-30 AA Cleavage site of Signal Peptidase Step 2: Signal Recognition Particle (SRP) is a ribonucleoprotein consisting of 6 proteins and and a small cytoplasmic RNA (scRNA) that is approximately 300 nt in length. SRP binds to the hydrophobic domain of signal sequence and pauses translation. Step 3: SRP/ribosome complex binds to the SRP receptor in the RER membrane. The SRP receptor has intrinsic GTPase activity that triggers release of SRP from the signal sequence. Step 4: Translocon is a transmembrane channel in the RER composed of 3 integral membrane protein subunits. The ribosome binds to the translocon upon release of SRP and the polypeptide is positioned in the channel. Translation resumes and growing polypeptide chain is translocated through the channel. Step 5: Signal Peptidase is an enzyme in the RER lumen that cleaves off the signal sequence from the amino terminus. When translation is complete, the secretory protein folds into its proper tertiary structure and is processed further as it moves through the RER and Golgi complex. Protein Synthesis & Secretion [6] C. ROUGH ENDOPLASMIC RETICULUM (RER) 2. Transmembrane Proteins Transmembrane proteins are produced in the RER and distributed to other membranes via intracellular transport vesicles. Transmembrane proteins have 1 or more membrane spanning Types(2) domains. These hydrophobic domains are inserted laterally into the RER membrane by the translocon during translation. There are 2 main types of transmembrane proteins. a) Single-pass: 1 transmembrane domain. Either the N- or C-terminus can extend into the cytosol with the other end extending into the lumen of the RER. b) Multi-pass: 2 or more transmembrane domains. Both N- and C- termini can extend into the cytosol or into the RER lumen. Alternatively, as shown here, the termini of multi-pass proteins can extend into opposite cellular compartments. CYTOSOL N C C N C N RER LUMEN 3. Synthesis of Single-pass Transmembrane Proteins • Single-pass proteins can be generated by means of an Internal Signal Sequence (ISS), which is an internal hydrophobic domain of 20-25 AA in length. The ISS functions as a Signal Sequence that binds to SRP and causes a pause in translation. mRNA 5´ 3´ 5´ N Translation in cytosol generates an ISS CYTOSOL 3´ Binding of SRP to ISS pauses translation N GDP•Pi ISS 5´ 3´ SRP • SRP/ribosome binds to the 3´ 5´ N 5´ N SRP receptor in the RER membrane. After SRP release, the ribosome binds to the translocon and the ISS is inserted into the channel. SRP receptor Translation resumes. RER LUMEN Translocon • The ISS also functions as a membrane-spanning domain. When translation resumes, the translocon releases the ISS laterally into the RER membrane. CYTOSOL • Translation continues on the 5´ 3´ cytosolic side of the RER to 5´ 3´ 5´ N complete polypeptide synthesis. N • Single-pass proteins can be inserted with N-terminus extending into the cytosol (top) or the SRP RER lumen (bottom). The AA receptor ISS fxn sequence of the ISS determines Translocon RER LUMEN how the protein is oriented in the translocon. 3´ N N C C 3´ N N Protein Synthesis & Secretion [7] C. ROUGH ENDOPLASMIC RETICULUM (RER) 4. Synthesis of Multi-pass Transmembrane Proteins (refer to schematic on next page) • Multi-pass proteins contain a series of transmembrane spanning domains derived from alternating ISS and Stop Transfer Sequences (STS) in the primary AA sequence. st • The 1 ISS in the polypeptide is synthesized during translation in the cytoplasm. ISS binds to SRP and pauses translation. The SRP/ribosome binds to the SRP receptor in RER membrane. • SRP is dissociated by receptor GTPase activity. The ribosome binds to the translocon and st inserts the 1 ISS. The AA sequence of the ISS determines whether the N-terminus extends into the cytoplasm or RER lumen. In this schematic, the N-terminus extends into RER lumen. • ISS is released laterally by the translocon into the RER membrane. Translation resumes and the polypeptide continues to lengthen as it passes through the translocon channel. • When the 1st STS is generated, translation pauses to allow lateral release into RER membrane. The ISS and STS are now transmembrane domains connected by a loop in the cytoplasm. nd • Translation resumes and a 2 ISS in the polypeptide is generated. SRP binds to the ISS and pauses translation. The SRP/ribosome binds to the SRP receptor in RER membrane. • SRP is dissociated by receptor GTPase activity. The ribosome binds to the translocon and inserts the 2nd ISS. nd • 2 ISS is released laterally by the translocon into the RER membrane. Translation resumes and the polypeptide lengthens as it passes through the translocon channel. nd • When the 2 STS is generated, translation pauses to allow lateral release by the translocon into the RER membrane. A second cytoplasmic loop has been produced. • Translation resumes. Additional membrane-spanning domains and loops can be generated until, as shown by this schematic, a stop codon terminates translation. Hydropathy Profile of Multi-pass Membrane Protein Graph In this plot of a multi-pass transmembrane protein, the shaded areas represent hydrophobicity of transmembrane domains approximately 20-25 AA in length along the entire primary sequence. AA # 5. Protein Modifications in the RER a) N-linked Glycosylation of Proteins: Oligosaccharide is added to Asn in the lumen of RER. • The entire oligosaccharide is transferred intact from the lipid carrier dolichol phosphate. • The reaction occurs co-translationally. • Other enzymes in the RER lumen further trim sugar residues in the oligosaccharide portion of the protein. 5´ 3´ 5´ CYTOSOL Fig ER membrane RER LUMEN Oligosaccharide transferase 3´ Protein Synthesis & Secretion [8] C. ROUGH ENDOPLASMIC RETICULUM (RER) 5. Protein Modifications in the RER Fig(2) b) Glycolipids: Addition of GPI Anchors (Glycosylphosphatidylinositol) • GPI anchors are synthesized on the luminal CYTOSOL side of the RER membrane. • The GPI anchor is added to specific proteins in the RER membrane that have a membranespanning domain near the C-terminus. This domain is cleaved and the newly created C-terminus of the protein is linked to the GPI anchor via the NH3+ group of ethanolamine. • GPI-anchored proteins are transported in vesicles that bud from the RER and fuse with the plasma membrane. Consequently, a GPIanchored protein extends out from the extracellular surface of the cell. RER LUMEN Y Plasma membrane Cytosol RER lumen Y Y Vesicle GPI-anchored protein 6. Protein Folding in the RER a) Chaperones: Proteins that assist in the proper folding of newly synthesized polypeptides into the correct tertiary structure. (2) Examples: CYTOSOL • BiP - Hsp70 chaperone protein that binds to polypeptide as it enters the RER lumen through BiP the translocon. Fig • Calnexin & Calreticulin- Proteins RER LUMEN in RER lumen that recognize oligosaccharide chains of N-linked glycoproteins. b) Protein Disulfide Isomerase (PDI): Enzyme in RER lumen that catalyzes correct formation of disulfide bonds between Cys residues- critical for proper protein folding. PDI N- N- N- N- RER lumen -C Incorrect disulfide bonds -C -C -C PDI Correct disulfide bonds 7. Protein Quality Control in the RER: Chaperones act as sensors of misfolded protein a) ER-Associated Degradation (ERAD): Misfolded proteins in the RER are transported to cytosol for degradation by the ubiquitin-proteasome system. b) Unfolded Protein Response: Stress response triggered by overload of unfolded proteins in the RER. A protein kinase pathway is activated that causes arrest of translation. D. SMOOTH ENDOPLASMIC RETICULUM (SER) Protein Synthesis & Secretion [9] TEM of Leydig cell in testis 1. Structure • SER is a branching network of membrane enclosed tubules without attached ribosomes. • SER is relatively sparse in many cell types, but abundant in cells active in metabolism of lipids such as steroid hormone producing cells. 2. Main Functions of SER a) Synthesis of Lipids: Lipids synthesized on cytosolic side of SER membrane b) Calcium Storage and Release: Well developed in striated muscle (sarcoplasmic reticulum) c) Glycogen metabolism: Glucose-6-phosphatase in liver SER d) Drug Detoxification: Cytochrome P450 in the SER of hepatocytes (liver) 3. Synthesis of Lipids: Phospholipids, Cholesterol and Ceramide a) Location: Synthesis of lipids occurs on the cytosolic side of SER membranes by enzymes in the cytoplasm. SER lumen b) Flippases: These enzymes catalyze flipping of lipids between the outer (cytosolic) layer and inner (luminal) layer of the membrane. CYTOSOL Cytosol Lumen Flippase Cytosol Plasma membrane cytosolic enzymes SER Membrane (4) Lumen SER LUMEN (4) Protein Synthesis & Secretion [10] E. GOLGI COMPLEX 1. Structure of Golgi Complex a) Stack of flattened membrane sacks (cisternae) b) Transport Vesicles: Shuttling of contents between membranes c) cis Face: Convex, usually oriented toward nucleus, also called entry face d) trans Face: Concave, also called exit face 2. Compartments of Golgi Complex a) cis Golgi Network b) Golgi stack (medial & trans) c) trans Golgi Network 3. Tubular Vesicular Network or Compartment: Intermediate between RER and the cis Golgi Network EM of Golgi Complex 4. Protein Processing in the Golgi Complex a) Modifications to N-linked oligosaccharides of glycoproteins: Ordered series of reactions in Golgi cisternae catalyzed by glycosyltransferases and glycosidases Protein Protein Processing Rx in Golgi: • Removal of Mannose • Addition of GlcNAc • Addition of Gal • Addition of Sialic Acid • Removal of mannose and addition of residues occur from cis to trans. • Occurs in plasma membrane and secreted proteins. = GlcNAc = Gal N-linked oligosaccharide in RER = N-acetylneuraminic acid b) Phosphorylation of N-linked oligosaccharides on lysosomal proteins Protein Protein • Occurs in the cis Golgi network. • Tags lysosomal enzymes with with mannose-6-P for sorting into lysosomes. Mannose-6-Phosphate Protein Synthesis & Secretion [11] E. GOLGI COMPLEX + H3N CH COO -O S 3 CH2 O CH2 d) Sulfation of Proteins • Sulfate group is added to Tyr residues. Residue Location • Sulfation occurs mainly in trans Golgi network. GlcNAc protein c) O-linked Glycosylation of Proteins Residue(3) • Sugar residues are added to Ser, Thr or Tyr. Location • Sugars are added one at a time, mainly in the cis & medial Golgi cisternae. protein 4. Protein Processing in the Golgi Complex e) Summary: Protein Processing in the Golgi Complex Table cis Golgi Network Phosphorylation of N-linked of oligosaccharides Golgi cisternae cis Modifications of N-linked Oligosaccharides: • Removal of Mannose • Addition of GlcNAc, Galactose & Sialic Acid Addition of O-linked Oligosaccharides trans trans Golgi Network Sulfation of proteins on tyrosine 5. Synthesis of Sphingomyelin and Glycolipids The enzymes that synthesize sphingomyelin and glycolipids from ceramide are located in Location the lumen of the Golgi cisterna. Glycolipids and sphingomyelin are made on the luminal surface of the membrane and move to other organelles by budding of vesicles from the Golgi cisternae. Plasma membrane Ceramide (made in ER) Glycolipids Luminal surface Cytosolic surface Sphingomyelin Golgi cisterna Luminal surface Protein Synthesis & Secretion [12] E. GOLGI COMPLEX 6. Secretory Pathways of trans Golgi Network: 3 Main Types a) Constitutive Secretion: Transport vesicles bud from the trans Golgi network in clathrincoated vesicles. The coat sheds and the transport vesicles deliver materials (proteins & lipids) in a continuous process to endosomes, plasma membrane and other organelles. • Some of these vesicles secrete products by exocytosis such as ECM components. • It is also called the default pathway. b) Regulated Secretion: Secretory products such as hormones, neurotransmitters and digestive enzymes are stored in membrane-bound secretory granules and released by exocytosis in response to a specific signal. • Secretory products are sorted and exit the trans Golgi network by budding of clathrincoated vesicles to form Immature Secretory Granules (ISG). • Excess membrane is recycled from ISGs back to the trans Golgi network. • After the coat is shed, secretory product becomes concentrated during development into a Mature Secretory Granule (MSG). Maturing granules exhibit a “bullseye” appearance. • Exocytosis of MSGs is regulated by specific signals. c) Lysosome Pathway: Lysosomal enzymes are tagged with mannose-6-P and exit the trans Golgi network by budding of clathrin-coated vesicles. • After the coat is shed, the vesicles carrying lysosomal enzymes fuse with late endosomes. These vesicles are sometimes referred to as primary lysosomes. • Subsequently, late endosomes mature into lysosomes. Fig (1-4) =Clathrin trans Golgi Network Clathrin-coated vesicle Immature Secretory Granule (ISG) Lysosome Pathway Late Endosome Mature Secretory Granule (MSG) Regulated Secretion Plasma Membrane Constitutive Secretion Lysosome Protein Synthesis & Secretion [13] F. VESICULAR TRAFFICKING 1. Anterograde and Retrograde Transport in the Golgi Complex Transitional RER RER Tub Ves Network Fig COPI COPII COPII COPI cis Golgi Network cis Cisterna COPI COPI medial Cisterna trans Cisterna trans Golgi Network Clathrin Late Endosome Lysosome 2. Coated Transport Vesicles: 3 Established Types Fxn Org(3) (2) a) COPI (Coat Protein 1): Retrograde transport of vesicles • Tubular Vesicular Network to RER • cis Golgi Network to RER • trans to cis between cisternae of Golgi stack b) COPII (Coat Protein II): Anterograde transport of vesicles • Transitional RER to Tubular Vesicular Network or cis Golgi network • Tubular Vesicular Network to cis Golgi Network c) Clathrin-coated vesicles: Transport in both directions between the trans Golgi network and other cellular compartments. Protein Synthesis & Secretion [14] F. VESICULAR TRAFFICKING 3. Protein Sorting into Transport Vesicles • Transmembrane proteins of budding vesicles contain a specific AA sequence that functions as a sorting sequence for export in a transport vesicle. • The vesicular coat contains Adaptor Protein (AP) that binds to a sorting sequence in a transmembrane protein and couples it to the protein coat of the budding transport vesicle. • Two families of small GTP-binding proteins (ARF and Rab) regulate activity of adaptor proteins during coat formation. Vesicular coat protein (COPI, COPII or Clathrin) Adapter protein (AP) binds to coat AA sorting sequence in cytosolic domain of the transmembrane receptor binds to adaptor protein AA sorting sequence in cytosolic domain of a transmembrane protein binds to adaptor protein Cargo protein in lumen binds to the transmembrane receptor Budding Transport Vesicle lumen a) Cargo Proteins: For sorting of cargo proteins in the lumen, i.e., secretory proteins and lysosomal enzymes, a transmembrane receptor binds to its cargo in the lumen and couples it to an adaptor protein via a sorting sequence in the cytosolic domain of the receptor. The cargo bound to receptor is then sorted into a coated vesicle. b) Transmembrane Proteins: The cytoplasmic domain of transmembrane proteins contains a sorting sequence that couples it to an adaptor protein for sorting into a coated vesicle. 4. Sorting Sequences for Specific Types of Transport Vesicles Sorting Sequence Transport Step Vesicle Coat Cargo Proteins KDEL TVN or cis Golgi to RER, trans to cis Golgi COPI Luminal RER KKXX TVN or cis Golgi to RER, trans to cis Golgi COPI RER Membrane Di-acidic (ex. DXE) RER to TVN or cis Golgi, TVN to cis Golgi COPII Membrane and Secreted Clathrin Lysosomal Mannose-6-P trans Golgi to the Late Endosome X = any Amino Acid, TVN = Tubular Vesicular Network Clathrin Fig COPII Tubular Vesicular Network (TVN) 200 nm Transitional RER COPI 100 nm Protein Synthesis & Secretion [15] F. VESICULAR TRAFFICKING 5. Budding of Transport Vesicles • A membrane GTPase called Dynamin is required to pinch off the budding transport vesicle. • After a transport vesicle buds off into the cytoplasm, the coat dissolves so that the vesicle can recognize and fuse with correct target membrane. Adaptor Protein Dynamin Coat Target Membrane Cargo Protein lumen Budding Transport Vesicle Transport Vesicle 6. Fusion of Transport Vesicles to Target Membranes Fusion steps(5) Rab •GTP v-SNARE Effector • Transport vesicle membrane contains v-SNARE proteins and effector proteins (sorting signals). • Rab•GTP is recruited to membrane and interacts with effector proteins. Diag steps Receptor t-SNARE • Target membrane contains receptors and t-SNARE proteins. •GTP •GDP Transport -> v-SNARE | Effector protein Target -> t-NARE | Receptor protein • Binding of effector protein to receptor(s) on the target membrane facilitates tethering via interactions of v-SNARE and t-SNARE proteins. • GTP hydrolysis and membrane fusion Protein Synthesis & Secretion [16] F. VESICULAR TRAFFICKING 7. Orientation of Membrane and Secreted Proteins During Cellular Transport Extracellular Space Plasma Membrane Y Cytosol Secretory protein: e.g. peptide hormone Integral membrane protein: Y e.g. cell surface receptor Transport Vesicle Y Y lumen Golgi cisternae Y Cytosol Transport Vesicle Y Transitional RER Y RER lumen 8. Secretory Granules and Exocytosis Note the bullseye appearance of the Mature Secretory Granules (MSGs). Extracellular space Extracellular space