Intracellular Compartments and Transport

Course Announcements

  • Quiz 3 closes today at 11:59 PM

  • Covers lectures 10, 11, and 13

Learning Outcomes of the Lecture

After this lecture you should be able to:

  • Locate sites of protein synthesis and understand how signaling sequences control protein localization.

  • Understand how membrane-bound organelles are divided between cells.

  • Identify three sorting localizations for proteins (cytosol, organelle, membrane/secreted) and the ribosome's location.

  • Compare and contrast three protein transport mechanisms: gated, transmembrane, and vesicular transport.

  • Describe how proteins are transported into the nucleus, mitochondria, and ER.

  • Differentiate the function and position of different transport signals, including post-transport cleavage.

Intracellular Compartments

  • Cytosol:

    • Contains many metabolic pathways; protein synthesis; the cytoskeleton

  • Nucleus:

    • Contains the main genome; DNA and RNA synthesis

  • Endoplasmic Reticulum (ER):

    • Synthesis of most lipids; synthesis of proteins for distribution

  • Golgi Apparatus:

    • Modification, sorting, and packaging of proteins/lipids

  • Lysosomes:

    • Intracellular degradation

  • Endosomes:

    • Sorting of endocytosed material

  • Mitochondria:

    • ATP synthesis via oxidative phosphorylation

  • Chloroplasts (in plant cells):

    • ATP synthesis and carbon fixation via photosynthesis

  • Peroxisomes:

    • Oxidative breakdown of toxic molecules

Eukaryotic Cells and Organelles

  • Basic membrane-enclosed organelles have specific functions dependent on compartmentalization, determined by lipid and protein composition.

  • Different eukaryotic cells have similar organelles but in varying numbers (e.g., more mitochondria in muscle cells compared to fat cells).

Intracellular Compartments Dynamics

  • Organelles like lysosomes and endosomes continuously undergo changes and transport molecules via endocytosis and exocytosis processes.

Protein Sorting (Targeting)

  1. Cytosolic Proteins:

    • Synthesized by free ribosomes, functioning in the cytosol.

  2. Organelle Proteins:

    • Synthesized by free ribosomes and transported to specific organelles.

  3. Secreted and Membrane Proteins:

    • Synthesized by ribosomes on the ER, then either secreted or embedded in cell membranes.

Mechanisms of Protein Import

1. Gated Transport

  • Involves nuclear pores allowing transport between the cytosol and the nucleus.

  • Nuclear proteins such as histones synthesized in the cytosol enter the nucleus through these pores.

2. Transmembrane Transport

  • Occurs from cytosol to organelles (mitochondria, chloroplasts, peroxisomes, ER).

  • Proteins enter in an unfolded state and depend on translocators in the organelle membranes.

3. Vesicular Transport

  • Transport between ER, Golgi, lysosomes, and for secretion.

Signal Sequences in Protein Transport

  • Proteins contain specific signal sequences or signal patches determining the target organelle, similar to a zip code.

  • Sequence examples (not to memorize specific amino acids):

    • Import into ER: Positive signal sequence

    • Nuclear Import: Nuclear localization signal (NLS)

    • Mitochondrial Import: Signal sequence at the N-terminus that is cleaved after import

Protein Transport to the Nucleus

  1. Nuclear protein with NLS interacts with import receptor.

  2. This complex engages with nuclear pore cytosolic fibrils.

  3. Movement occurs through a gel-like nuclear pore, allowing entry into the nucleus where the cargo is released.

The Structure of the Nucleus

  • Contains nuclear pore complexes made of >50 proteins.

  • Small molecules (< 5000 daltons) pass freely; larger proteins require signals for import.

Transport into Mitochondria and Chloroplasts

  • Precursor proteins kept unfolded in cytosol by chaperones and utilize specific receptors for entry through translocators.

  • Import involves signal and stop-transfer sequences for anchoring in membranes.

Targeting Proteins to the Endoplasmic Reticulum (ER)

  • Protein translocation is co-translational (occurs while the protein is synthesized).

  • ER signal sequence at N-terminus is recognized by the signal recognition particle (SRP), which directs the ribosome to the ER membrane for translocation.

Homework Questions

  • Explore the fate of a protein containing both an ER signal sequence and a nuclear localization sequence.

  • Create a comparison table for the three types of protein transport (nucleus, mitochondria/chloroplasts, ER).