In-Depth Notes on Intracellular Transport

Introduction to Intracellular Transport

• Focus of today's lecture is intracellular transport mechanisms in eukaryotic cells.
• Key topics to be covered include:
  • Overview of protein sorting and distribution
  • Importance of intracellular organelles
  • Transport mechanisms (vesicular transport, nuclear pores, translocators)

Review and Upcoming Lectures

• Next lectures will include:
  • Tuesday: continued discussion on intracellular transport
  • Science Galvan
  • Lecture on the cell cycle, followed by cancer-related discussions
  • Final exam will only cover new content, not cumulative.

Key Organelles Involved

• Focus will primarily be on the following organelles:
  • Endoplasmic Reticulum (ER)
  • Golgi apparatus
  • Plasma membrane

Protein Sorting

• Definition: Distribution of proteins to specific organelles.
• Importance of protein sorting in maintaining cellular function and health.
• Mislocalization of proteins can lead to diseases.
• Notable diseases associated with transport mechanisms (e.g., genetic mutations).

Signal Sequences

• Signal Sequence: A peptide sequence within the protein that facilitates transport.
  • Import Sequence: Directs proteins into organelles.
  • Export Sequence: Directs proteins out of organelles.
• Length of signal sequences can range from 3 to 60 amino acids.

Transport Mechanisms

Overview of Three Strategies

1. Vesicular Transport:
   • Use of vesicles (lipid bubbles) to transport proteins between organelles.
   • Example: Protein movement from ER to Golgi.
2. Nuclear Pore Complex (NPC):
   • Selective transport of molecules into and out of the nucleus through the nuclear pore.
   • Requires a nuclear localization signal (NLS) for import.
   • Example of cargo: RNA, proteins.
3. Translocators:
   • Complete protein transport across membranes (e.g., mitochondria and ER).

Nuclear Pore Complex (NPC)

• Nuclear Pore Complex:
  • Composed of proteins called nucleoporins that form a selective barrier between cytoplasm and nucleus.
  • Facilitates import and export of proteins and RNA.
• Mechanism:
  • Requires nuclear import receptors to recognize NLS.
  • RAN protein involved: Converting RAN-GDP to RAN-GTP to facilitate transport.
  • Protein import involves a shuttling mechanism – cargo is recognized and transported in both directions.

Nuclear Import Mechanism

• Cargo Shuttling:
  • RAN-GDP binds to nuclear import receptor and cargo.
  • Upon translocation across the NPC, RAN-GTP is introduced, leading to cargo release.
  • RAN-GAP converts RAN-GTP back to RAN-GDP to continue cycling.

Translocators: Mitochondria

• Mitochondria:
  • Possess two membranes: outer and inner.
• Transport mechanism:
  • Protein targeting to mitochondria involves a mitochondrial import sequence.
  • Initial binding to mitochondrial import receptors followed by translocation through translocators (outer and inner).
  • Proteins often need to be unfolded to pass through translocators.
  • Proteins are refolded by chaperones once inside the mitochondrial matrix.

Translocators: Endoplasmic Reticulum (ER)

• Endoplasmic Reticulum (ER):
  • Rough ER: Studded with ribosomes.
• Key process:
  • SRP (Signal Recognition Particle) captures nascent polypeptides and transports them to the ER translocator.
  • Protein enters the ER lumen or may integrate into the ER membrane.
  • Proteins may possess additional signals for trafficking to the Golgi or termination in the ER.

Importance of Signal Peptidase

• Signal peptidase removes initial signal sequences after protein entry to reveal further targeting signals for downstream processes.

Start-Stop Transfer Sequences

• Proteins destined for the ER membrane may utilize start-stop transfer sequences.
• Mechanism of cutting and revealing additional sequences for correct localization.

Conclusion and Review

• Reinforcing mechanisms of intracellular transport, including lessons learned from examples of organelles successfully transporting proteins.
• Upcoming topics will further elaborate on vesicular transport, specific molecular mechanisms, and their relevance in health and disease.
• Emphasis on maintaining a continuous cycle of protein synthesis and turnover in eukaryotic cells.

Next Class Preparations

• Come prepared with knowledge on previous lectures and questions regarding transport mechanisms to clarify and deepen understanding.