vation of cdc25 tyrosine phosphatases by B-type 
cyclins: evidence for multiple roles of mitotic cy- 
clins. Ce// 67:1181-1194. 
Giordano, A., Lee, J.H., Scheppler, J.A., Herrmann, 
C, Harlow, E., Deuschle, U., Beach, D., and 
Franza, B.R.,Jr. 1991. Cell cycle regulation of his- 
tone HI kinase activity associated with adenoviral 
protein ElA. Science 253:1271-1275. 
Jans, D.A., Ackerman, M.J., Bischoff, J.R., Beach, 
D. H., and Peters, R. 1991. p34'^'^'^^-mediated 
phosphorylation at T'^* inhibits nuclear import of 
SV-40 T antigen proteins. / Cell Biol 115:1 203- 
1212. 
Jessus, C, and Beach, D. 1992. Oscillation of MPF 
is accompanied by periodic association between 
cdc25 and cdc2-cyclin B. Cell 68:323-332. 
Matsumoto, T., and Beach, D. 1991. The spil 
GTPase interacts with RCCl in cell cycle depen- 
dency. Cold Spring Harb Symp Quant Biol 
56:385-398. 
Xiong, Y., and Beach, D. 1991 . Population explo- 
sion in the cyclin family. Curr Biol 1:362-364. 
Xiong, Y., Menninger, J., Beach, D., and Ward, D. 
1992. Molecular cloning and chromosomal 
mapping of human D-type cyclins. Genomics 
13:575-584. 
MEMBRANE-CYTOSKELETON INTERACTIONS 
G. Vann Bennett, M.D., Ph.D., Investigator 
Dr. Bennett's laboratory has focused on the spec- 
trin-based membrane skeleton, a system of proteins 
associated with plasma membranes of most animal 
cells. Spectrin and its associated proteins are candi- 
dates to play a role in localization of integral mem- 
brane proteins at specialized regions of the plasma 
membrane. Physiologically important cell domains 
involving spectrin include axons of neurons, the 
neuromuscular junction, nodes of Ranvier of myelin- 
ated axons, and basolateral domains of epithelial 
cells. 
Research in the past year has addressed basic ques- 
tions related to association of spectrin with the 
plasma membrane. Much of the work involves an- 
kyrins, which are a family of spectrin-binding pro- 
teins that link the spectrin skeleton to multiple 
membrane proteins, including ion channels such as 
the voltage-dependent sodium channel and the 
Na^.K"^ ATPase. Progress in the past year has resulted 
in the discovery that a major class of ankyrin-binding 
proteins in adult brain are members of an immuno- 
globulin (Ig) superfamily of neural cell adhesion 
molecules, detailed analysis of the structure and reg- 
ulation of ank}'rins, and complete cloning and 
sequencing of cDNA encoding the subunit of 
spectrin. 
Brain Ankyrins 
Antibodies, cDNA cloning, and mutant mice have 
been used to distinguish three different ankyrin 
genes in the nervous system. One ankyrin isoform is 
localized at nodes of Ranvier and initial axonal 
segments, sites known to contain the voltage- 
dependent sodium channel. A specific interaction of 
this isoform of ankyrin with the sodium channel may 
play an important role in the morphogenesis and/or 
maintenance of the node of Ranvier. This ankyrin is 
likely to represent a distinct gene product, since it 
persists in mutant mice missing a form of ankyrin 
located in cell bodies and does not cross-react with 
antibodies against the major form of brain ankyrin. A 
research goal has been to identify the gene encoding 
this nodal/axonal hillock isoform of ankyrin. The 
cDNAs that encode a new ankyrin closely related to 
the major isoform of ankyrin in brain have been iso- 
lated. The possibility that the new protein repre- 
sents the form of ankyrin at the node of Ranvier will 
be evaluated. 
The major form of ankyrin in brain occurs as ei- 
ther a 220-kDa polypeptide or a 440-kDa form with 
a 220-kDa inserted sequence due to alternative 
splicing of pre-mRNA. The 440-kDa brain ankyrin is 
the first ankyrin detected in developing brain, with 
a peak at postnatal day 10 that subsequently falls 
two- to threefold in adult brain. The 440-kDa neona- 
tal ank}Tin is present in neuronal processes includ- 
ing unmyelinated axons and disappears from axons 
following myelination. This protein thus is a candi- 
date to perform a specialized role in unmyelinated 
axons. The predicted configuration of the inserted 
sequence of 440-kDa ankyrin is an ~200-nm ran- 
dom coil. Portions of the inserted sequence that 
have been expressed in bacteria have such an ex- 
tended, nonglobular configuration. The 440-kDa 
ankyrin therefore is shaped like a ball and chain, 
with membrane and spectrin-binding activities lo- 
20 
