Molecular motors and polymerization are the two most common mechanisms for powering cell movement. However, a third type of engine, based on a spring, is recognized as a third type of cellular engine. Based on its ability to store energy in polymeric filaments, springs are responsible for thefastest and most powerful cell movements. The most dramatic example of a spring-based mechanism is the msec contraction of the mm-long stalk of Vorticella. We have been studying another example of a spring, the coiled bundle of actin filaments in a Limulus sperm. In unactivated sperm, a bundle of overtwisted actin filaments encircles the base of the nucleus. When the sperm contacts the egg, the coil untwists and straightens to its full 60 pm length in a few seconds. As it straightens, the bundle expends 10(-9) ergs of energy and generates 3 pN of force at the tip. However, extension occurs without the aid of ATP hydrolysis by myosin or actin polymerization. Instead, fluctuation in actin twist is captured as conformational energy in the coiled bundle. Calcium relaxes the over twist by opening the conformation of scruin. As the filaments untwist, the bundle straightens. The long term goal of this renewal is to describe the fundamental principles and properties of cellular springs through studies of an actin spring.
Our specific aims are the following: 1. To describe how calcium unlatches actin twist through a conformation change in scruin; 2. To uncover the mechanism that localizes the conformation change at the base of the nucleus; 3. To measure how chemical and mechanical energy contributes to the acrosome reaction. These goals will test and refine the hypothesis that actin twist drives the extension of the actin bundle. Studies of the acrosome reaction are basic to understanding the role of the protein polymers in causing cell movements such as cell crawling, nuclear migrations, and bacterial motility. These processes are fundamental to embyronic development, cancer, infection and immunity.
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