GTP Binding Proteins and Nuclear Transport Mechanisms

Active vs. Inactive States
- Proteins can exist in either active or inactive states depending on whether they are bound to GTP (active) or GDP (inactive).
- Binding and hydrolysis of GTP results in a conformational change in the protein structure.
GTP Binding Protein Family
- General family name: GTP binding proteins.
- Human cells possess approximately 100 types of GTP binding proteins.
Example: RAS Protein
- This protein serves as a classic example among GTP binding proteins.
- Notable structure resembles other proteins in the family, typically appearing as rounded ‘blobs’.
- Functions as an enzyme: transforms between bound states (GDP vs. GTP) that dictate activity level.

Phosphorylation Overview
- Distinct from the effects of GTP and GDP binding, phosphorylation involves the addition of phosphate (P) groups to specific amino acids in proteins.
- Key amino acids modified by phosphorylation include:
- Serine
- Threonine
- Tyrosine
Enzymes Involved
- Kinases: Enzymes that catalyze the addition of phosphate groups to proteins.
- Phosphatases: Enzymes that remove phosphate groups, thus reversing the action of kinases.

Kinases can be classified into:
- Serine/Threonine Kinases
- Tyrosine Kinases
Corresponding phosphatases:
- Serine/Threonine Phosphatases
- Tyrosine Phosphatases
- Chemical Nature of Phosphate
The phosphate group is small but is negatively charged (-2 in proteins).
Phosphates can induce new ionic interactions in proteins, altering their activity.
- Amino Acid Targeting
Kinases have consensus recognition motifs for phosphorylation: similar structures surrounding the target amino acid facilitate recognition.
Example: Serine and Threonine kinase targets posited in similar environments due to their common hydroxyl (-OH) groups.

Examples
- PKA: A serine/threonine kinase with specific substrates.
- ERK: Another serine/threonine kinase; displays preference based on recognition motifs, but not necessarily limited to exact sequences.
General Mechanism of Action
- Phosphorylation transforms enzyme structure, revealing or hiding active sites critical for function.
- ATP serves as a substrate donating phosphate during phosphorylation, leading to conformational shifts that enhance or reduce enzymatic activities.

Allosteric Regulation via cAMP
- Example: PKA (Protein Kinase A) exists in an inactive heterotetramer complex composed of regulatory and catalytic subunits.
- Binding of cyclic AMP (cAMP) to regulatory subunits induces a conformation change, activating the enzyme by releasing catalytic subunits.

Function
- The proteasome is a large cellular complex responsible for degrading unneeded or damaged proteins.
- Proteins tagged for degradation typically undergo ubiquitination.
Structure
- The proteasome resembles a barrel with a central core filled with proteolytic enzymes.
- Two lid complexes recognize ubiquitin-tagged proteins, preventing non-target proteins from entry.
Ubiquitin and Its Role
- Ubiquitin: a small protein used as a tag for protein degradation.
- Proteins are tagged with multiple ubiquitin molecules to signal for degradation within the proteasome.

Involves three enzyme classes: E1, E2, and E3 ligases.
- E1 (Ubiquitin Activating Enzyme): Activates ubiquitin.
- E2 (Ubiquitin Conjugating Enzyme): Transfers ubiquitin from E1 to E3.
- E3 (Ubiquitin Ligases): Recognizes target proteins and attaches ubiquitin.
Results in polyubiquitination, facilitating recognition by the proteasome.

Nuclear Envelope
- Composed of two membranes (inner and outer) with a perinuclear space between them, continuous with the endoplasmic reticulum (ER).
Nuclear Lamina
- Provides structural support to the nucleus and consists of intermediate filament proteins (lamins).
- Lamins maintain nuclear shape and anchor chromatin near the inner membrane, aiding in gene expression regulation.
Diseases Related to Nuclear Lamina
- Hutchinson-Gilford Progeria Syndrome (HGPS): A genetic disorder caused by mutations in the lamin gene, leading to premature aging.

Nuclear Import
- Proteins that need to enter the nucleus possess nuclear localization signals (NLS) recognized by importin receptors.
- Importins bind NLS, allowing proteins to pass through the nuclear pore complex (NPC) into the nucleus.
Ran GTPase
- A small GTP binding protein (Ran) is pivotal in regulating nuclear transport.
- Ran is active when bound to GTP (in the nucleus) and inactive when bound to GDP (in the cytoplasm).

Ran GTP interacts with importin to release cargo proteins into the nucleus.
After unloading, the importin-Ran GTP complex exits the nucleus, where Ran GTP is hydrolyzed by Ran GAP, releasing the importin for reuse.
- Nuclear Export
Proteins destined to exit the nucleus possess nuclear export signals recognized by exportin receptors.
Ran GTP is also central in this export process, facilitating protein release from exportin and translocation through the NPC.
- Exportins and Karyopherins
Karyopherins describe the family of proteins that include importins (for import) and exportins (for export).

Nuclear transport is highly regulated, with specificity dictated by nuclear localization and export signals.
Ran GTP interactions play a crucial role in determining the fate of proteins as they are transported into and out of the nucleus.

Understanding these molecular processes is key to grasping how proteins are regulated, especially in relation to signaling pathways.
- Knowledge of these mechanisms highlights the complexity of cellular regulations and their implications in various biological functions and diseases.