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J. Cell Biol.,
Volume 139, Number 3, November 3, 1997 683-693
Department of Cell Biology, The Scripps Research Institute, La Jolla, California 92037
Brush border myosin-I (BBM-I) is a single-headed myosin found in the microvilli of intestinal epithelial cells, where it forms lateral bridges connecting
the core bundle of actin filaments to the plasma membrane. Extending previous observations (Jontes, J.D.,
E.M. Wilson-Kubalek, and R.A. Milligan. 1995. Nature
[Lond.]. 378:751-753), we have used cryoelectron microscopy and helical image analysis to generate three-dimensional (3D) maps of actin filaments decorated
with BBM-I in both the presence and absence of 1 mM
MgADP. In the improved 3D maps, we are able to see
the entire light chain-binding domain, containing density for all three calmodulin light chains. This has enabled us to model a high resolution structure of BBM-I
using the crystal structures of the chicken skeletal muscle myosin catalytic domain and essential light chain.
Thus, we are able to directly measure the full magnitude of the ADP-dependent tail swing. The ~31° swing
corresponds to ~63 Å at the end of the rigid light
chain-binding domain. Comparison of the behavior of
BBM-I with skeletal and smooth muscle subfragments-1 suggests that there are substantial differences in the
structure and energetics of the biochemical transitions
in the actomyosin ATPase cycle.
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