Intracellular Compartments and Transport

Course Overview

  • Course Title: BSC 360
  • Lecture: Intracellular Compartments and Transport Part 1
  • Instructor: Dr. Nikki Camlin, PhD
  • Contact: Nicole.Camlin@usm.edu
  • Textbook Reference: Essential Cell Biology Chapter 15

Learning Outcomes

After this lecture, you should be able to:

  • Identify locations of protein synthesis and understand signaling sequences for protein localization.
  • Understand the distribution of membrane-bound organelles in cells.
  • Describe and differentiate the three sorting localizations for proteins: cytosol, organelle, and membrane/secreted proteins.
  • Compare and contrast the three mechanisms of protein transport: gated transport, transmembrane transport, and vesicular transport.
  • Describe the process of protein transport into the nucleus, mitochondria, and ER, including transport signals and whether they are cleaved after transport.

Intracellular Compartments

Main Functions of Membrane-Enclosed Organelles in Eukaryotic Cells:

  • Cytosol: Contains metabolic pathways; protein synthesis; supports the cytoskeleton.
  • Nucleus: Contains the genome; site for DNA and RNA synthesis.
  • Endoplasmic Reticulum (ER): Synthesis of lipids and proteins; distributes proteins to organelles and the plasma membrane.
  • Golgi Apparatus: Modifies, sorts, and packages proteins and lipids for secretion or delivery to other organelles.
  • Lysosomes: Intracellular degradation.
  • Endosomes: Sorting of endocytosed material.
  • Mitochondria: ATP synthesis via oxidative phosphorylation.
  • Chloroplasts (in plant cells): ATP synthesis and photosynthesis.
  • Peroxisomes: Oxidative breakdown of toxic molecules.

Eukaryotic Cell Membrane Organelles

  • Cells share basic organelles but vary in quantity (e.g., number of mitochondria in muscle vs. fat cells).
  • Each organelle performs similar functions across cell types, emphasizing the role of compartmentalization in cellular efficiency.

Mechanisms of Protein Sorting

Types of Proteins and Their Destinations:

  1. Cytosolic proteins: Synthesized by free ribosomes 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 and transported to the cell membrane or secreted.

Protein Transport Mechanisms:

  1. Gated Transport:

    • Transport between cytosol and nucleus via nuclear pores.
    • Nuclear proteins (e.g., histones) synthesized in the cytosol must enter the nucleus through these pores.
    • RNA produced in the nucleus is exported to the cytoplasm through nuclear pores.

  2. Transmembrane Transport:

    • Occurs from the cytosol to organelles like mitochondria, chloroplasts, and peroxisomes.
    • Requires translocator proteins on organelle membranes.
    • Proteins are kept unfolded during transport.

  3. Vesicular Transport:

    • Transport between vesicles such as the ER, Golgi apparatus, lysosomes, and for secretion from the cell.

Protein Transport Signals

Signal Sequences and Patches:

  • Signal Sequence: Short amino acid sequence (15-60 aa) that directs a protein to its destination.
  • Signal Patch: Formed from amino acids in different segments of the protein, acting like a zip code for targeting.
  • Importance of signal sequences for correct localization—altering a sequence can change a protein's destination.

Typical Signal Sequences:

  • Import into ER: Contains hydrophobic residues.
  • Import into mitochondria: Includes mitochondrial targeting sequences at the N-terminus.
  • Nuclear Import: Nuclear localization signal (NLS) can be anywhere in the protein, with bipartite configurations possible.

Current Transport Mechanisms Overview:

  1. Nuclear Transport:

    • Bi-directional transport through nuclear pores; large proteins utilize NLS for entry.
    • Nuclear pore complexes facilitate passage of molecules < 5000 daltons.

  2. Mitochondrial/Chloroplast Transport:

    • Precursor proteins kept unfolded by chaperone proteins during cytosol transport.
    • Sequential signal sequences required for proper targeting to compartments within these organelles.

  3. ER Transport:

    • Co-translational translocation: As protein is synthesized, it is simultaneously translocated into the ER lumen.
    • The ER signal sequence is typically cleaved during translation, with further modifications (like glycosylation) occurring post-translocation.

Exam Preparation Questions

  1. A protein with an ER signal sequence at its N-terminus and a nuclear localization sequence in its middle—what would happen?

    • Consider: Location upon entering the cell, the interaction with transport machinery, and potential pathways based on signal sequence nature.

  2. Create a table summarizing the three transport mechanisms with respect to:

    • Synthesis location
    • Signal sequence location
    • Nature of transport (post-translational vs co-translational)
    • State of transported proteins (folded or unfolded)
    • Fate of the signal sequence after transport