Substructure of the outer dynein arm.

Area of Science:

• Cytoskeletal motor proteins
• Cilia and flagella structure
• Microtubule dynamics

Background:

• Outer dynein arms are essential motor proteins responsible for ciliary and flagellar movement.
• Understanding their substructure is key to elucidating their mechanical function.
• Previous studies have provided limited insights into the detailed arrangement of outer dynein arm components.

Purpose of the Study:

• To analyze the detailed substructure of the outer dynein arm.
• To investigate the conformational changes of the outer dynein arm in different functional states (rigor vs. relaxed).
• To propose models linking outer dynein arm morphology to its function in microtubule-based motility.

Main Methods:

• Quick-frozen deep-etch replica electron microscopy of Tetrahymena and Chlamydomonas axonemes.
• Analysis of axonemes in rigor (ATP-depleted) and relaxed (ATP- or vanadate plus ATP-treated) states.
• Observation of living cilia in Tetrahymena cells.

Main Results:

• The outer dynein arm comprises five distinct components: head, two feet, stalk, and linker.
• Specific interactions were identified: feet with A-microtubule, stalk with B-microtubule.
• Significant spatial differences were observed between rigor and relaxed states, with the stalk altering its B-microtubule affinity in the relaxed state, which is also adopted in vivo.

Conclusions:

• A detailed five-component model for the outer dynein arm substructure is presented.
• Conformational changes, particularly involving the stalk, are crucial for dynein function.
• The findings provide a structural basis for understanding how outer dynein arms generate force and movement in cilia and flagella.