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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 3 peopleNoteStudied by 14 peopleNoteStudied by 18 peopleNoteStudied by 26 peopleNoteStudied by 38 peopleNoteStudied by 62 people
Untitled
NoteStudied by 3 peoplePhotosynthesis Quiz
NoteStudied by 14 peoplePLTW Medical Interventions Unit 2:
NoteStudied by 18 peopleChapter 4- Elements, Compounds and Mixtures
NoteStudied by 26 peopleThe Integumentary System
NoteStudied by 38 peopleChapter 2 - Diversity in organizations
NoteStudied by 62 people