Motor Proteins

Introduction to Cell Motility

  • Definition: Cell motility encompasses movement within the cell, cell contraction, and cell crawling.

Key Concepts of Cell Crawling

  • Protrusion of the Cell: This involves the extension of the cell in the direction of movement through the growth of the actin cytoskeleton.

  • Cell Contraction: Occurs at the rear of the cell where the cell pulls itself forward by contracting.

  • Movement of Molecules: Motor proteins facilitate the movement of cellular cargo from the front to the back of the cell. This is achieved through vesicles that are walked along microtubules.

Motor Proteins Involved in Cell Motility

Kinesins

  • Function: Kinesins are motor proteins that transport cargo along microtubules.

  • Directionality: Kinesins move toward the plus end of the microtubule.

  • Structure:

    • Globular Head: Connects to the microtubule and functions like feet for movement.

    • Coiled Coil Region: Spring-like, flexible, absorbs tension during movement.

    • Cargo Binding Region: Where kinesins bind to vesicles or organelles to transport them.

Mechanism of Kinesin Movement

  • Three-Step Process:

    1. The kinesin has one foot (front) bound to ADP and the other foot (back) bound to ATP.

    2. Upon ATP hydrolysis of the back foot, it releases from the microtubule.

    3. ATP binds to the front foot, causing an allosteric change that swings the linker region forward.

    4. The back foot shifts to become the front foot, now bound to ADP, preparing for the cycle to repeat.

Myosins

  • Function: Myosins are motor proteins that bind to actin filaments and facilitate contraction.

  • Cell Contraction Mechanism: Myosins pull actin filaments closer together, resulting in the shortening of the cell.

Structure of Myosins

  • Has a domain that binds to microfilaments and a cargo carrying end.

  • In muscle cells, myosins interact with actin filaments organized in a sarcomere.

Mechanism of Myosin Action

  1. Myosin binds to the actin filament with its head group.

  2. The addition of ATP causes an allosteric change, releasing myosin from the actin filament.

  3. ATP hydrolysis pulls myosin into the cocked position (resembles pulling back a catapult).

  4. As the phosphate is released from myosin, tension is released, and myosin swings back to grab the actin filament.

  5. The power stroke occurs as myosin pulls the actin filament in a forward direction, leading to contraction of the cell and shortening overall dimensions.

Summary of Motor Protein Functions

  • Kinesins and myosins work to move cargo within the cell.

  • Kinesins transport vesicles along microtubules, while myosins interact with actin filaments to contract the cell.

  • The coordinated effort of these motor proteins facilitates effective cell movement, characterized by the protrusion and contraction mechanisms driven by the actin cytoskeleton.