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J. Cell Biol.,
Volume 141, Number 6, June 15, 1998 1393-1406

* Department of Cell Biology, Health Sciences Center, University of Virginia, Charlottesville, Virginia 22908; We have investigated the function of
p55CDC, a mammalian protein related to Cdc20 and
Hct1/Cdh1 in Saccharomyces cerevisiae, and Fizzy and
Fizzy-related in Drosophila. Immunofluorescence studies and expression of a p55CDC-GFP chimera demonstrate that p55CDC is concentrated at the kinetochores
in M phase cells from late prophase to telophase. Some
p55CDC is also associated with the spindle microtubules and spindle poles, and some is diffuse in the cytoplasm. At anaphase, the concentration of p55CDC at the kinetochores gradually diminishes, and is gone by
late telophase. In extracts prepared from M phase, but
not from interphase HeLa cells, p55CDC coimmunoprecipitates with three important elements of the M
phase checkpoint machinery: Cdc27, Cdc16, and Mad2.
p55CDC is required for binding Mad2 with the Cdc27
and Cdc16. Thus, it is likely that p55CDC mediates the
association of Mad2 with the cyclosome/anaphase-promoting complex. Microinjection of anti-p55CDC antibody into mitotic mammalian cells induces arrest or delay at metaphase, and impairs progression of late
mitotic events. These studies suggest that mammalian
p55CDC may be part of a regulatory and targeting
complex for the anaphase-promoting complex.
Amgen, Inc.,
Thousand Oaks, California 91320; and § Department of Biology, Gilmer Hall, University of Virginia, Charlottesville, Virginia
22901
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