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© The Rockefeller University Press, 0021-9525/1997//683 $5.00
The Journal of Cell Biology, Volume 139, Number 3, , 1997 683-693

Article

Brush Border Myosin–I Structure and ADP-dependent Conformational Changes Revealed by Cryoelectron Microscopy and Image Analysis



James D. Jontes and Ronald A. Milligan

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.

Abbreviations used in this paper: AM, actomyosin; BBM-I, brush border myosin–I; CaM, calmodulin; CTF, contrast transfer function; ELC, essential light chain; HC, heavy chain; LCBD, light chain–binding domain; 3D, three-dimensional.

Address all correspondence to R.A. Milligan, Department of Cell Biology, The Scripps Research Institute, 10550 North Torrey Pines Road, La Jolla, CA 92037. Tel.: (619) 784-9827. Fax: (619) 784-2749. E-mail: milligan @scripps.edu


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