Eukaryotic Cell Structure and Function: Nuclear Transport, Endomembrane System, and Cytoskeleton

Cell Structure Overview

Introduction to Cell Biology

Cell biology is the study of cells, integrating microscopy and biochemical analysis to understand cellular components and their functions. The structure of each cell component is closely related to its function, and the size and number of organelles vary according to the cell's specialized role.

• Fat cells: Rounded, globular structures specialized for storing lipids.

• Cardiac muscle cells: Long and tapered, adapted for contraction and movement.

• Organelle content varies with cell type and function.

Cell Fractionation

Cell fractionation is a laboratory technique used to separate cellular components based on size and density, often using differential centrifugation.

• Cell lysis breaks open cells to release organelles.

• Differential centrifugation separates components by spinning at high speeds, generating forces up to 1,000,000 x G.

• Ultracentrifuges can spin at 130,000 rpm to isolate small particles.

Dynamic Nature of Cells

Cells are highly dynamic, with rapid molecular movement and turnover.

• Human cells use approximately 10 million ATP molecules per second.

• Enzymes catalyze over 25,000 reactions per second.

• Membrane phospholipids can traverse the organelle or cell in under a minute.

• Mitochondria are replaced about every 10 days.

Nuclear Transport

Structure and Function of the Nucleus

The nucleus is the information center of eukaryotic cells, where genetic information in DNA is decoded and processed. Large suites of enzymes interact to produce RNA messages.

• Nucleolus: Site of ribosome assembly.

• Ribosomal RNA (rRNA): Binds proteins to form ribosomes.

• Messenger RNA (mRNA): Carries genetic information for protein synthesis.

Structure and Function of the Nuclear Envelope

The nuclear envelope separates the nucleus from the cytoplasm and is perforated with nuclear pore complexes.

• Nuclear pore complexes: Openings that connect the nucleus with the cytosol; consist of about 30 different proteins.

Transport Through the Nuclear Envelope

Transport into the nucleus is highly selective and regulated.

• Inbound traffic includes nucleoside triphosphates, proteins for DNA replication, RNA synthesis, and ribosome assembly.

• A typical cell imports over 500 molecules through 2000 nuclear pores every second.

Nuclear Localization Signals (NLS)

Proteins destined for the nucleus contain a nuclear localization signal (NLS), which acts as a molecular address for transport through the nuclear pore complex.

• NLS: Short amino acid sequence that allows proteins to enter the nucleus.

• Example: Pyruvate kinase with NLS is transported into the nucleus.

Endomembrane System

Overview and Function

The endomembrane system manufactures, ships, and recycles cellular materials. Proteins are actively imported into organelles from the cytosol, each with a specific molecular zip code for correct delivery.

• Acid hydrolases: Enzymes shipped to lysosomes for digestion.

Secretory Pathway and Pulse-Chase Experiment

Organelles participate in a secretory pathway, starting in the rough endoplasmic reticulum (RER) and ending with product secretion.

• Pulse-chase experiment: Tracks protein movement using radiolabeled amino acids.

• After the pulse, proteins are in the RER; during the chase, they move to the Golgi apparatus and secretory vesicles.

• Conclusion: RER and Golgi function together in the endomembrane system.

Signal Hypothesis for Protein Targeting

Proteins bound for the endomembrane system have an ER signal sequence that directs them to the RER.

• ER signal sequence: 20-amino-acid-long zip code.

• Steps: Ribosome synthesizes signal sequence, binds to SRP, moves to ER membrane, protein is fed into ER lumen, and signal sequence is removed.

Protein Modification and Transport

Proteins in the ER lumen are folded and may undergo glycosylation (addition of carbohydrate groups), forming glycoproteins. These modifications are signals for further transport.

• Proteins are transported in vesicles from ER to Golgi apparatus.

• Golgi apparatus is dynamic, with cisternal maturation and compartment-specific enzymes.

• Proteins are tagged for lysosome delivery (e.g., mannose-6-phosphate tag).

• Exocytosis: Bulk transport of macromolecules out of the cell.

Lysosome Recycling Pathways

Lysosomes digest large molecules and recycle cellular materials via three main pathways:

• Receptor-mediated endocytosis: Specific uptake of molecules via receptors.

• Phagocytosis: Engulfment of large particles or cells.

• Autophagy: Recycling of damaged organelles within the cell.

• Pinocytosis: Non-specific uptake of extracellular fluid.

Dynamic Cytoskeleton

Overview and Functions

The cytoskeleton is a dense, complex network of fibers that provides structural support, motility, and regulation of biochemical processes. It is dynamic, allowing rapid changes in cell shape and movement.

• Mechanical support: Maintains cell shape and organelle position.

• Motility: Enables cell movement and intracellular transport.

• Regulation: Conveys signals from outside to inside the cell.

Types of Cytoskeletal Elements

• Actin filaments (microfilaments)

• Intermediate filaments

• Microtubules

Actin Filaments (Microfilaments)

Actin filaments are the smallest cytoskeletal elements, composed of globular actin proteins. They resist tension forces and are involved in various cellular processes.

• Maintain and change cell shape

• Muscle contraction

• Cytoplasmic streaming

• Cell motility (pseudopodia formation)

• Cell division (cleavage furrow formation)

Actin filaments have plus and minus ends; the plus end grows faster. Movement is powered by myosin motor proteins using ATP.

• Cytokinesis

• Cytoplasmic streaming

• Cell crawling via pseudopodia

Intermediate Filaments

Intermediate filaments are fibrous proteins coiled into cables, providing structural stability. Keratins are a well-known family of intermediate filaments.

• Maintain cell shape

• Anchor nucleus and organelles

• Compose nuclear lamina

• More stable and less dynamic than other filaments

Microtubules

Microtubules are the largest cytoskeletal elements, made of α- and β-tubulin dimers. They are dynamic and serve as tracks for organelle and vesicle movement.

• Maintain cell shape

• Cell motility (cilia and flagella)

• Chromosome movement during cell division

• Organelle movement

• Resist compression forces

Microtubules originate from the microtubule organizing center (MTOC), called the centrosome in animal cells, which contains centrioles arranged in a 9-3 pattern.

Motor Proteins and Vesicle Transport

Motor proteins such as kinesin and dynein convert chemical energy (ATP) into mechanical movement along microtubules.

• Kinesin: Moves vesicles toward the plus end of microtubules.

• Dynein: Moves toward the minus end and is involved in cilia/flagella movement.

Cilia and Flagella

Structure and Function

Cilia and flagella are hairlike projections that enable cell movement. Prokaryotic and eukaryotic flagella differ in structure and movement mechanisms.

• Prokaryotic flagella: Helical rods made of flagellin, rotate like a propeller.

• Eukaryotic flagella: Surrounded by plasma membrane, whip back and forth.

• Cilia: Short, hairlike projections, similar in structure to flagella but more numerous.

Axoneme Structure

Most cilia and flagella have a "9 + 2" arrangement of microtubules called the axoneme.

• 9 microtubule doublets surround 2 central microtubules.

• Axoneme originates from the basal body, structurally identical to a centriole.

Movement Mechanism

Dynein motor proteins form arms between microtubule doublets, using ATP to "walk" along microtubules and cause bending, resulting in swimming motion.

• Dynein moves toward the minus end, causing linked doublets to bend.

• Only one side of the axoneme moves at a time, producing coordinated bending.

Summary Table: Cytoskeletal Elements

Element	Structure	Main Functions
Actin Filaments	Two intertwined strands of actin	Cell shape, motility, muscle contraction, division
Intermediate Filaments	Fibrous proteins coiled into cables	Structural stability, nuclear lamina, anchoring organelles
Microtubules	Hollow tubes of tubulin dimers	Cell shape, vesicle/organelle movement, cilia/flagella, chromosome movement

Key Equations and Terms

• ATP hydrolysis (energy for motor proteins): $\text{ATP} \rightarrow \text{ADP} + \text{P}_i$

• Glycosylation (formation of glycoproteins): $\text{Protein} + \text{Carbohydrate group} \rightarrow \text{Glycoprotein}$

Additional info: These notes expand on the original slides by providing definitions, examples, and a summary table for cytoskeletal elements, as well as key equations relevant to cell biology.