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© The Rockefeller University Press,
0021-9525/1998//155 $5.00
The Journal of Cell Biology, Volume 141, Number 1,
, 1998 155-162
Regular Articles |
Three-dimensional Structure of Acanthamoeba castellanii Myosin-IB (MIB) Determined by Cryoelectron Microscopy of Decorated Actin Filaments
Department of Cell Biology and Anatomy, The Johns Hopkins University School of Medicine, Baltimore, Maryland 21205
The Acanthamoeba castellanii myosin-Is were the first unconventional myosins to be discovered, and the myosin-I class has since been found to be one of the more diverse and abundant classes of the myosin superfamily. We used two-dimensional (2D) crystallization on phospholipid monolayers and negative stain electron microscopy to calculate a projection map of a "classical" myosin-I, Acanthamoeba myosin-IB (MIB), at
18 Å resolution. Interpretation of the projection map suggests that the MIB molecules sit upright on the membrane. We also used cryoelectron microscopy and helical image analysis to determine the three-dimensional structure of actin filaments decorated with unphosphorylated (inactive) MIB. The catalytic domain is similar to that of other myosins, whereas the large carboxy-terminal tail domain differs greatly from brush border myosin-I (BBM-I), another member of the myosin-I class. These differences may be relevant to the distinct cellular functions of these two types of myosin-I. The catalytic domain of MIB also attaches to F-actin at a significantly different angle, 10°, than BBM-I. Finally, there is evidence that the tails of adjacent MIB molecules interact in both the 2D crystal and in the decorated actin filaments.
Abbreviations used in this paper: 2D, two-dimensional; 3D, three- dimensional; BBM-I, brush border myosin-I; GPA, glycine-proline-alanine; LCBD, light chain–binding domain; MIB, Acanthamoeba myosin-IB; SH3, src-homology domain-3.
J.D. Jontes would like to thank B. Carragher (Beckman Institute, University of Illinois, Urbana-Champaign, IL) for access to the Philips CM200TEM and for her hospitality during collection of the MIB helical data.
J.D. Jontes's present address is Department of Molecular and Cellular Physiology, Stanford University School of Medicine, Stanford, CA 94305.
E.M. Ostap's present address is Department of Physiology, University of Pennsylvania School of Medicine, Philadelphia, PA 19104.
T.D. Pollard's present address is The Salk Institute for Biological Studies, La Jolla, CA 92037.
Address all correspondence to R.A. Milligan, Department of Cell Biology, MB25, The Scripps Research Institute, 10550 North Torrey Pines Road, La Jolla, CA 92037. Tel.: (619) 784-9827. Fax: (619) 784-2749. E-mail: milligan{at}scripps.edu
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