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04.15.25_ module 7 Actin and Microtubule Polymerization
- Biological Polymers:
- Examples: Actin, tubulin, microtubules.
- Subunits have structural polarity: Plus end and minus end.
- Polarity: Different structural characteristics on different ends of polymers.
- Subunit Incorporation:
- Subunits undergo structural changes when incorporated into polymers.
- Growth dynamics differ at the plus and minus ends due to polarity.
- Essential for understanding microtubule and actin polymerization behavior.
- Critical Concentration:
- Actin: - Critical concentration measured at 2 µM.
- If concentration is above critical: Both ends grow, but plus end grows faster.
- If concentration is below critical: Generally leads to shrinking of both ends.
- Microtubules: Similar behavior to actin, but with distinct rates and dynamics.
- Dynamic Behavior of Polymers:
- Plus end: More dynamic, grows and shrinks faster.
- Minus end: Less dynamic, grows and shrinks slower.
- Ongoing processes: Plus end more likely to bond ATP, minus end loses ADP upon disassembly.
- Nucleotide Binding:
- Actin binds ATP, tubulin binds GTP.
- Hydrolysis rates are slow, requiring additional factors (like GAPs) to increase efficiency.
- Critical Concentrations in Polymers:
- Plus end critical concentration lower (0.1 µM) than minus end (0.7 µM).
- Polarity leads to differences in polymerization dynamics and critical concentrations.
- Treadmilling Phases:
- At intermediate concentrations (0.1-0.7 µM), a steady state is achieved between growth at the plus end and shrinkage at the minus end.
- Overall filament length remains stable while individual filaments undergo treadmilling.
- Actin in Cells:
- Cells maintain a high concentration of ATP-bound actin to facilitate polymerization.
- Other regulatory proteins control actin dynamics (e.g., sequestering actin, promoting nucleation).
- Historical Context of Actin & Tubulin:
- Actin recognized through studies on muscle structure and contraction (thin filaments).
- Microtubules identified through polarized light microscopy, revealing dynamic behavior during cell division.
- Techniques evolved to study these structures in living cells, including the use of fluorescent tags.
- Microtubule Structure and Dynamics:
- Comprised of alpha and beta tubulin dimers, forming protofilaments in a tubular structure.
- Microtubule dynamics involve phases of rapid growth (polymerization) and rapid shrinking (depolymerization) known as dynamic instability.
- Experimental Validation of Dynamic Behavior:
- Research distinguished between actin treadmilling and microtubule dynamic instability.
- Various experimental techniques (e.g., fluorescence microscopy, reconstitution assays) lead to a better understanding of cellular mechanics.
- Stiffness in Polymers:
- Microtubules: More rigid than actin filaments, with higher persistence lengths (5200 microns for microtubules vs. 17.7 microns for actin).
- The persistence length determines how much bending can occur under thermal motion.
- Key Experimental Findings:
- Studies showed that microtubules behave differently at steady state compared to actin filaments, supporting the concept of dynamic instability versus treadmilling in actin.
- Cellular Parameters:
- Cellular factors influence actin and tubulin availability, ensuring proper polymerization dynamics in response to physiological needs.
- Conclusions:
- Understanding the polymerization dynamics of actin and microtubules is critical for elucidating their roles in cellular structure and function.
- Ongoing research explores the complex regulation of these cytoskeletal components to ensure cellular integrity and response to environmental signals.
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Explore Top NotesNoteStudied by 20 peopleNoteStudied by 37 peopleNoteStudied by 27 peopleNoteStudied by 16 peopleNoteStudied by 26 peopleNoteStudied by 3503 people
The Haber Process
NoteStudied by 20 peopleSociological Perspective on Race
NoteStudied by 37 peopleCubic and Reciprocal Graphs
NoteStudied by 27 peopleMarket Supply and Demand Externalities
NoteStudied by 16 peoplePhysical Science - Chapter 2
NoteStudied by 26 peopleBoolean Expressions and If Statements
NoteStudied by 3503 people