Transmembrane proteins, Golgi and COP part 1
Synthesis of Transmembrane Proteins
Introduction
Topic of discussion: Synthesis of transmembrane proteins
Related topics: Golgi complex, vesicular transport, coatomers
Suggested study questions to solidify understanding of concepts discussed in previous lectures
Previous Lectures Recap
Discussion on transport vesicles:
Definition: Small membrane-bound sacs that transport proteins (both soluble and membrane proteins) within the cell or out of the cell.
Examples: Transport to Golgi apparatus or plasma membrane.
Overview of the endoplasmic reticulum (ER):
Structure: Interconnected network of flattened sacs and tubules.
Functions:
Protein and lipid synthesis.
Calcium storage.
Detoxification processes.
Comparison of rough ER vs. smooth ER:
Rough ER:
Studded with ribosomes.
Key indicator when observed via electron microscopy: presence of dotted ribosomes.
Functions:
Protein synthesis for secretion or membrane insertion.
Smooth ER:
Lacks ribosomes.
Functions:
Lipid synthesis.
Detoxification.
Calcium storage.
Emphasized overlap of functions between rough and smooth ER.
Mutations and Protein Translation Issues
Discussion on mRNA mutations:
Importance of capping mRNA strands: Mutations can disrupt protein translation and trafficking, leading to diseases.
Translation steps:
mRNA carries genetic code from DNA to ribosome.
Translation begins at AUG start codon (Codes for methionine).
Ribosome reads codons sequentially until it reaches a stop codon, signaling the end of translation.
Normal polypeptide chain structure:
Starts with an amino group (NH2) and ends with a carboxyl group (COOH).
Proper sequence is crucial for correct protein folding and targeting (to ER, mitochondria, nucleus).
Mutation Effects:
Example of mutation site in mRNA as single nucleotide polymorphism (SNP):
Types of mutations:
Premature stop codon: Results in shortened protein.
Missense mutation: Changes one amino acid, potentially altering protein function.
Frameshift mutation: Caused by insertion or deletion leading to shifted reading frame.
Consequences of mutations:
Defective targeting of misfolded/shortened proteins.
Potential loss of function or gain of function in proteins, disrupting cell signaling or metabolism.
Links to diseases like cystic fibrosis, leukocyte adhesion deficiencies, Tay-Sachs disease, etc.
Organelle Focus: Peroxisomes
Definition: Peroxisome is a small membrane-bound structure found in almost all eukaryotic cells including plants, animals, and fungi.
Also termed microbodies.
Function:
Breakdown of very long chain fatty acids.
Detoxification of harmful molecules (like hydrogen peroxide), which protects cells from oxidative damage.
Synthesis of specific phospholipids, notably plasmologens, essential for brain and lung function, impacting nerve cell membranes and myelin.
Overview of Zellweger syndrome:
Type: Peroxisomal trafficking disorder.
Cause: Mutations in the peroxisome assembly factor (e.g., PEX1) impairing biogenesis and function of peroxisomes.
Results in absence of functional peroxisomes, leading to:
Accumulation of toxic metabolites.
Impaired production of plasmologens.
Clinical manifestations of the syndrome:
Profound neurological defects (potentially leading to seizures).
Developmental delays.
Renal dysfunction.
Hepatomegaly (liver enlargement).
Craniofacial abnormalities.
Consequences of Protein Trafficking Issues
Understanding protein trafficking and localization issues leading to diseases, such as:
Cystic Fibrosis (CF):
Caused by mutations in the CFTR gene (cystic fibrosis transmembrane conductance regulator).
Function: CFTR protein is a chloride ion channel in epithelial cell membranes regulating salt and water balance.
Common mutation: delta F508 (deletion of three nucleotides causing loss of phenylalanine at position 508), resulting in a misfolded CFTR protein.
Consequences of CFTR misfolding:
Misfolded proteins targeted for degradation rather than transported to cell surface.
Lack of functional CFTR on epithelial cell surfaces leads to accumulation of thick mucus, chronic lung infections, digestive problems, and decreased organ function.
Summary of Protein Synthesis Process
Overview of cotranslational import process for soluble proteins into the ER:
Ribosome Translation:
Ribosome begins translating mRNA in the cytoplasm.
Growing polypeptide chain emerges containing signal sequence (a stretch of hydrophobic amino acids).
Recognized by signal recognition particle (SRP), preventing protein synthesis in the wrong compartment.
Docking:
SRP directs ribosome to SRP receptor on rough ER membrane.
Translocon Interaction:
Ribosome binds to ER channel (translocon) and translation continues with SRP released, threading polypeptide into ER lumen.
Signal sequence cleaved by signal peptidase once translation is complete.
Post-Translation Processing:
Proteins are either retained in ER lumen or transported to Golgi apparatus for further modification.
Overview of synthesis of transmembrane proteins:
Distinction from soluble proteins; transmembrane proteins have a signal anchor sequence recognized during translation.
Translation and insertion into the lipid bilayer occurs cotranslationally, with hydrophobic regions embedding into the membrane.
Example of Type I single-pass transmembrane proteins:
Translation begins, SRP recognizes signal sequence, directs to ER.
Ribosome docks at translocon; translation resumes.
Translation stops when the stop transfer anchor sequence is encountered, anchoring protein in membrane.
Final complete protein orientation established with N terminus in ER lumen and C terminus in cytosol.
Post-Translational Import and Mitochondria/Chloroplasts
Proteins destined for mitochondria or chloroplasts are synthesized completely in cytoplasm:
Post-translational import process with distinct signal sequences.
Encounter receptor complexes on organelle membranes for translocation.
Translocase complexes guide proteins across inner and outer membranes.
Chloroplasts use TOC and TIC complexes similar to mitochondrial transport processes.
Overview of import process:
Proteins properly targeted and imported based on specific signal sequences, subsequently cleaved upon entry.
Golgi Apparatus Functions
The Golgi acts as a central post office for the cell:
Modifies, sorts, and packages proteins and lipids received from the ER for delivery to various destinations (another organelle, plasma membrane, secretion).
Structure: Stack of flattened membrane sacs called cisternae, with distinct functional regions:
Cis face (entry) towards the ER, receiving vesicles.
Trans face (dispatch) facing the plasma membrane to send vesicles out.
Movement through the stack includes processing like glycosylation and other modifications.
Proteins packaged into vesicles for intended destinations, including lysosomes and plasma membrane.
Different pathways:
Constitutive secretory pathway: Continuous transport of proteins.
Regulated secretory pathway: Controlled release of proteins depending on specific signals.
Vesicular Transport and Coating Proteins
Coat proteins (COP I, II, clathrin) play crucial roles in vesicle formation and trafficking:
COP II: Mediates transport from ER to Golgi (anterograde).
COP I: Manages retrograde transport from Golgi back to ER.
Clathrin: Involved in post-Golgi trafficking, endocytosis, and targeting vesicular transport.
Functions of coat proteins:
Forms vesicles by pulling membranes to create buds.
Selects cargo by interacting with specific receptors targeting proteins and lipids for transport.
Recognition of the importance of coat proteins ensures proper cargo be delivered to the right locations within the cell, ensuring cellular functionality and health.