NuMA associates with microtubule motors during mitosis to perform an essent
ial role in organizing microtubule minus ends at spindle poles. Using immun
ogold electron microscopy, we show that NuMA is a component of an electron-
dense material concentrated at both mitotic spindle poles in PtK1 cells and
the core of microtubule asters formed through a centrosome-independent mec
hanism in cell-free mitotic extracts. This NuMA-containing material is dist
inct from the peri-centriolar material and forms a matrix that appears to a
nchor microtubule ends at the spindle pole. In stark contrast to convention
al microtubule-associated proteins whose solubility is directly dependent o
n microtubules, we find that once NuMA is incorporated into this matrix eit
her in vivo or in vitro, it becomes insoluble and this insolubility is no l
onger dependent on microtubules. NuMA is essential for the formation of thi
s insoluble matrix at the core of mitotic asters assembled in vitro because
the matrix is absent from mitotic asters assembled in a cell-free mitotic
extract that is specifically depleted of NuMA. These physical properties ar
e consistent with NuMA being a component of the putative mitotic spindle ma
trix in vertebrate cells. Furthermore, given that NuMA is essential for spi
ndle pole organization in vertebrate systems, it is likely that this insolu
ble matrix plays an essential structural function in anchoring and/or stabi
lizing microtubule minus ends at spindle poles in mitotic cells. Cell Motil
. Cytoskeleton 42:189-203, 1999. (C) 1999 Wiley-Liss, Inc.