Protein Folding and Chaperones

Study Notes

Posttranslational Modifications and Protein Function

Proteins undergo posttranslational modifications that lead to functional activation, subcellular localization, secretion, or changes in activity/stability
A protein's fate post-translationally depends on its structure, including sequence and conformation, which determine its interactions with modifying enzymes and cellular localization signals

Cystic Fibrosis (CF)

Cystic fibrosis is an autosomal recessive disease prevalent in Caucasians, occurring at a frequency of about 1 in 2000
The CF gene is 230 kb long and contains 27 exons that encode a 1480 amino acid protein called cystic fibrosis transmembrane conductance regulator (CFTR)
CFTR is a cyclic AMP-regulated chloride channel with specific structural domains: two membrane spanning domains with six transmembrane regions each, two ATP-binding domains, and a regulatory domain with phosphorylation sites
CFTR's folding pathway involves interactions with chaperones, and mutations can disrupt this process

Effects of CF Mutations

CF mutations lead to defective electrolyte transport, affecting organs like the lungs, pancreas, and liver
Thick mucous secretions cause chronic lung disease and infections, posing life-threatening risks
In about 70% of CF patients, a deletion mutation affects phenylalanine 508, disrupting CFTR folding in the endoplasmic reticulum (ER), leading to degradation instead of proper transport to the cell surface
Lower temperatures or chemical chaperones like glycerol can aid mutant CFTR folding and function in cultured cells
Therapeutic strategies include drugs promoting chaperone interactions with mutant CFTR and aiding its folding and membrane transport, offering potential treatments for this form of cystic fibrosis

Protein Folding and Chaperones

Many proteins can fold into their native conformation spontaneously, while others require assistance from chaperones
Chaperones are proteins that reversibly bind to hydrophobic regions of nascent proteins or those in intermediate folding stages
Chaperones aid proteins in achieving their functional conformation, stabilize intermediates, promote correct folding, and prevent misfolding or aggregation
Chaperones can maintain proteins in an unfolded state for membrane passage, help unfold misfolded segments, and prevent inappropriate interactions with other proteins
Chaperones may use ATP hydrolysis to carry out their functions, and failure to fold correctly often leads to rapid protein degradation
Accumulation of misfolded proteins can lead to aggregation and contribute to serious diseases

Protein Export and Secretory Pathway

Proteins destined for export or specific organelles like the plasma membrane, lysosomes, endosomes, or Golgi apparatus are synthesized on rough endoplasmic reticulum (ER)-bound ribosomes
These proteins have a hydrophobic signal peptide near the amino terminus, typically consisting of a positively charged N-terminal region, a hydrophobic core of 8-12 amino acids in an α-helix, and a polar C-terminal segment serving as a cleavage site
Translation starts on cytosolic ribosomes, and as the signal peptide emerges (15-30 amino acids), it binds to a signal recognition particle (SRP), halting translation
The SRP, composed of six proteins and a small RNA molecule, binds to the signal peptide and GTP, forming a complex that moves to the ER
The SRP then binds to an SRP receptor on the ER membrane, leading the ribosome to a translocon, a receptor that facilitates protein passage through the membrane

Description

Understand the role of chaperones in protein folding, including their assistance in achieving native conformation and stabilization of intermediates. Learn about spontaneous folding and how chaperones aid in correct folding.