title: Calcium regulation of an actin spring.
abstract: Calcium is essential for many biological processes involved in cellular motility. However, the pathway by which calcium influences motility, in processes such as muscle contraction and neuronal growth, is often indirect and complex. We establish a simple and direct mechanochemical link that shows how calcium quantitatively regulates the dynamics of a primitive motile system, the actin-based acrosomal bundle of horseshoe crab sperm. The extension of this bundle requires the continuous presence of external calcium. Furthermore, the extension rate increases with calcium concentration, but at a given concentration, we find that the volumetric rate of extension is constant. Our experiments and theory suggest that calcium sequentially binds to calmodulin molecules decorating the actin filaments. This binding leads to a collective wave of untwisting of the actin filaments that drives bundle extension.
authors.affiliation: Department of Physics, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA.
authors.forename: Barney K
authors.initials: BK
authors.lastname: Tam
authors.forename: Jennifer H
authors.initials: JH
authors.lastname: Shin
authors.forename: Emily
authors.initials: E
authors.lastname: Pfeiffer
authors.forename: P
authors.initials: P
authors.lastname: Matsudaira
authors.forename: L
authors.initials: L
authors.lastname: Mahadevan
mesh_terms: D000199:Actins / Q000737:chemistry / Q000502:physiology*; D000818:Animals; D002118:Calcium / Q000737:chemistry / Q000502:physiology*; D002478:Cells, Cultured; D055119:Elastic Modulus; D006737:Horseshoe Crabs; D008297:Male; D008954:Models, Biological; D008956:Models, Chemical; D020409:Molecular Motor Proteins / Q000737:chemistry / Q000502:physiology*; D013081:Sperm Motility / Q000502:physiology*; D013094:Spermatozoa / Q000737:chemistry / Q000502:physiology*; D013314:Stress, Mechanical
chemical_list: D000199:Actins; D020409:Molecular Motor Proteins; D002118:Calcium
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