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J. Cell Biol.,
Volume 141, Number 1, April 6, 1998 155-162
* Department of Cell Biology, The Scripps Research Institute, La Jolla, California 92037; and 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.
Department of Cell Biology and
Anatomy, The Johns Hopkins University School of Medicine, Baltimore, Maryland 21205
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