Protein Folding and Chaperones

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