Post-Translational Modifications, Regulation of Gene Expression & Protein Targeting

Levels of Gene Expression Regulation

  • Genetic instructions are converted from DNA into functional proteins through four primary levels of control:   - Epigenetic: Modifications to DNA or chromatin structure.   - Transcriptional: Control of mRNA production rates.   - Translational: Regulation of protein assembly.   - Post-translational: Covalent modifications to proteins after synthesis.

Epigenetic Regulation Mechanisms

  • DNA Methylation: DNA methyltransferases (DNMTS) add methyl groups (−CH3-CH_3) to cytosine bases, typically at CpG islands near gene promoters, to repress gene expression.

  • Histone Modification:   - Histone Acetyltransferases (HATs) add acetyl groups to lysine, neutralizing positive charges to relax chromatin into euchromatin, allowing transcription.   - Histone Deacetylases (HDACs) remove acetyl groups to restore positive charges and tight packing into heterochromatin, silencing transcription.

Transcriptional Control Elements

  • Promoters: The TATA box serves as the core site where general transcription factors recruit RNA Polymerase II to form the basal transcription complex.

  • Specific Factors: Activator or repressor proteins bind to regulatory sequences to modulate expression rates beyond basal levels.   - Enhancers: Increase transcription rates significantly; can be located far from the target gene.   - Silencers: Binding sites for repressor proteins that decrease transcription.

Protein Targeting and The Signal Hypothesis

  • Proteins contain intrinsic address labels called signal sequences, typically encompassing 1515 to 6060 amino acids.

  • Targeting Specifics:   - Endoplasmic Reticulum (ER): Identified by a hydrophobic core.   - Nucleus: Identified by basic amino acids.   - Mitochondria: Identified by an amphipathic helix.   - Cytosol: The default destination for proteins lacking a signal sequence.

Mechanisms of Protein Translocation and Transport

  • Co-Translational Translocation: The Signal Recognition Particle (SRP) binds the N-terminal signal sequence during translation, pauses the process, and docks at the SRP receptor on the Rough Endoplasmic Reticulum (RER). The polypeptide enters the RER lumen via a translocon.

  • Vesicular Transport:   - COPII-coated vesicles: Handle anterograde transport from the RER forward to the Golgi apparatus.   - COPI-coated vesicles: Manage retrograde transport of escaped ER-resident proteins from the Golgi back to the RER.

Lysosomal Sorting and I-cell Disease

  • Sorting Mechanism: Enzymes destined for lysosomes receive a Mannose-6-phosphate (M6P) carbohydrate tag in the cis-Golgi. M6P receptors in the trans-Golgi network package these enzymes into specific transport vesicles.

  • Clinical Correlation: I-cell disease (Mucolipidosis II) results from a genetic defect in the enzyme that adds the M6P tag. Lysosomal enzymes are mistakenly secreted, leading to the accumulation of waste in large inclusion bodies (I-cells).

Post-Translational Modifications (PTMs)

  • Phosphorylation: A reversible switch where Protein Kinases add phosphate groups from ATP to Serine, Threonine, or Tyrosine residues. Protein Phosphatases remove them to reverse the effect.

  • Glycosylation:   - N-linked: Sugars attach to Asparagine; begins in the RER.   - O-linked: Sugars attach to Serine or Threonine; occurs in the Golgi.

  • Proteolytic Cleavage: Irreversible activation where enzymes cut inactive precursors (zymogens) into functional forms, such as the processing of Proinsulin into active Insulin.

  • Ubiquitination: Target proteins are tagged with Ubiquitin chains and directed to the Proteasome for degradation into reusable amino acids.