Cell Cytoskeleton Lecture Notes
Midterm Review
- Midterm is this Wednesday.
- Format is similar to midterm one.
- Bring pen or pencil only; no calculators are allowed.
- The test comprises three problems.
- Content covers material up to, but not including, today's lecture.
- Focus is on material covered since midterm one.
- Foundational knowledge from before midterm one is assumed (e.g., knowing what homozygous means).
- Problems will be similar to recent homework assignments.
Cell Cytoskeleton
- Cells have a skeleton called the cytoskeleton, which is crucial for structure.
Overview
- The lecture will cover the cell cytoskeleton.
- The cytoskeleton provides structure to cells, similar to how bones provide structure to our bodies.
Components
- The three primary cytoskeletal proteins are:
- Microtubules (green).
- Actin (red).
- Intermediate filaments (blue).
- Intermediate filaments are abundant in the nucleus and around it.
- Microtubules run from around the nucleus to the cell surface.
- Actin filaments are concentrated near the cell surface.
Cell Size and Shape
- Scale bar: 10 microns.
- Cells in the body are typically more rounded, whereas cells on a hard substrate (e.g., glass) tend to spread out.
- Typical cell size: 10-20 microns.
- A micron (micrometer) is one-millionth of a meter (10^{-6} m).
Function
- The cytoskeleton provides structure and shape, similar to our skeleton.
- Unlike our skeleton, the cytoskeleton can provide force itself.
- Essential for cell movement and cell division (moving components to specific locations).
Components
- Three primary components:
- Actin.
- Smallest (6 nanometers).
- Stiff.
- Intermediate filaments.
- Intermediate size.
- Floppy, like rope.
- Microtubules.
- Largest (25 nanometers).
- Stiff.
- All three components are polymers made of smaller protein subunits.
Polymers
- Cytoskeletal elements are genetically encoded.
- Instead of encoding a long protein rod, cells encode smaller pieces that self-assemble into larger structures.
- Advantages include efficiency in genome usage and quicker assembly/disassembly.
- Dynamic nature allows cells to change shape rapidly, which is important for various functions.
- Unlike bones, which are static, cytoskeletal elements are dynamic.
Microtubules
- Largest of the cytoskeletal elements.
- Made of alpha and beta tubulin subunits.
- Subunits assemble to form a hollow cylindrical tube.
- The tube has a distinct alpha end and beta end (front and back).
Dynamics
- Dynamic nature is important for cell division and cell migration.
Cell Division
- The mitotic spindle, made of microtubules, appears suddenly at the beginning of M phase.
- Microtubules quickly assemble to separate chromosomes and then disassemble after division.
- The cycle of assembly and disassembly is continuous.
Cell Migration
- Microtubules help cells move by extending and grabbing.
- Cell shape changes dynamically during cell migration.
- Act as "highways" for vesicle transport (e.g., ER to Golgi).
- Highways must be able to reach the cell surface, even as it moves.
Growth and Shrinkage
- Microtubules grow slowly and shrink catastrophically.
- While growing, tubulin is bound to GTP, making it sticky.
- GTP converts to GDP over time.
- If GTP disappears completely from the tip, the microtubule becomes unstable and collapses.
- Length vs. time plot: growth is slow (tens of minutes), collapse is rapid (1-2 minutes).
- Microtubules extend and retract, searching for cellular components.
- Cells can change direction quickly by reorganizing the cytoskeleton.
Stabilization
- Capping proteins stabilize microtubules by capping the GTP head.
- Even when GTP converts to GDP, the microtubule remains stable.
Neurons
- Neurons have long axons with stable microtubules running along them.
- Microtubules transport proteins and neurotransmitters from the nucleus to the axon terminus.
- Capping proteins stabilize these microtubules.
Dynamic Instability (Video)
- Microtubules grow from the centrosome but can suddenly stop and shrink rapidly.
EB1 Protein (Video)
- EB1 binds to the GTP-tubulin cap at the growing ends of microtubules.
- Growing microtubules have GTP-tubulin caps; shrinking ones do not.
- Narrower than microtubules (10 nanometers).
- Not hollow; have twisted fibers.
- Floppy, like ropes, not stiff rods.
- Connect to proteins at cell borders, forming a strong network.
- Provide tensile strength to sheets of cells (e.g., skin, intestines).
- Prevent cells from falling apart when pulled.
Nuclear Lamins
- Line the inner surface of the nucleus, providing structural strength.
Actin
- Important for force application and contraction.
- Narrowest of the three cytoskeletal proteins (6 nanometers).
- Has a plus end and a minus end due to the organization of actin molecules.
G-Actin and F-Actin
- Individual actin molecules are called G-actin (globular actin).
- Actin fibers are called F-actin (filamentous actin).
- G-actin molecules self-assemble to form F-actin fibers.
Polymerization
- Initially, there is only G-actin, so there is no F-actin.
- Nucleation occurs when two G-actin molecules randomly stick together, initiating fiber growth.
- Elongation occurs as the fiber grows longer and longer.
- Eventually, a steady state is reached where there is no more net F-actin growth.
- Similar to boiling water, where bubble nucleation must occur before bubbles can grow.
Steady State
- At high G-actin concentrations, the chemical reaction proceeds forward to form F-actin.
- As more F-actin forms, the G-actin concentration decreases.
- At a certain point, the G-actin concentration is low enough that the F-actin starts to fall apart at the same rate as it is growing, resulting in a steady state.
ATP Exchange
- The process is similar to GTP in microtubules.
- Actin grows with an ATP cap.
- ATP is hydrolyzed to ADP, making the actin less sticky.
- Actin falls apart on the minus end and grows on the plus end.
- G-actin can be reactivated with ATP to bind on the plus end and grow.
Treadmilling
- Actin fibers can be free-standing, growing on the plus end and falling apart on the minus end simultaneously.
- Treadmilling refers to laying down tracks in front and picking them up behind.
- Actin grows on the plus end and shrinks on the minus end at the same rate, so the length of the fiber remains constant, but it moves forward.
Role in Cell Movement
- Actin treadmilling is important for cell movement.
- The cell extends arms forward, pushed out by actin growth.
- Actin falls apart on the back end, and G-actin diffuses back to the front.
- Myosin, another protein, sits on actin and crawls along it, causing contraction.
- Contraction pulls the rear of the cell forward.
- Reaching is controlled by treadmilling, and pulling is controlled by actin-myosin contraction.
Role in Cell Division
- Actin and myosin form a contractile ring that pinches off the cell during M phase.
- Actin and myosin slide against each other to cause contraction.
Muscle Contraction
- Muscle contraction is a result of actin and myosin sliding against each other.
- The details of muscle contraction are covered in BME 120.
Critical Concentration
- The critical concentration is the concentration of G-actin at steady state.
- At steady state, the amount of G-actin reaches a critical concentration.
- Rate of assembly = rate of disassembly.
- K{on} \times [G-actin] = K{off}
- [Critical Concentration] = \frac{K{off}}{K{on}}
- If [G-actin] > critical concentration, then assembly occurs.
Treadmilling and Growth Rates
- Plus end grows faster than the minus end because ATP makes G-actin stickier.
- At a certain G-actin concentration, the plus end grows, but the minus end shrinks.
- At the treadmilling state, the growth rate at the plus end equals the shrinking rate at the minus end, so the length of the actin fiber does not change.
- The actin moves along in space.
- This material will be in a future homework, but will not be on the midterm this Wednesday.
