Cell Fate Choices in the Nervous System and Elsewhere 
plasmic and appears to be influenced in its sub- 
cellular localization by the expression of Notch. 
While these observations are compatible with the 
notion that Notch interacts with a cytoplasmic 
protein, recent findings regarding both En- 
hancer of split and the intracellular domain of 
th&^Notch protein raise the possibility that Notch 
could directly interact with nuclear components. 
Most of the evidence supporting this hypothe- 
sis stems from our analysis of human Notch group 
homologues. In our attempts to dissect the mo- 
lecular mechanisms involving Notch group 
members, we have been exploring their involve- 
ment in vertebrate development. While helping 
to establish the degree to which the Drosophila 
paradigm is valid for vertebrate species, this work 
also permits us to make use of the vertebrate 
model's experimental advantages, such as better- 
defined cultured cell lines. 
The Drosophila Enhancer of split locus was 
shown to be defined genetically by several tran- 
scripts. With one exception, the genes contain 
the helix-loop-helix (HLH) motif found in a vari- 
ety of transcriptional regulators, and they are par- 
tially redundant functionally. The single gene 
that does not contain the HLH motif displays ho- 
mology to the |S-subunit of the signal-transducing 
G proteins. Cloning of human homologues, 
which were named TLEs (for transducin-like En- 
hancer of split), revealed an extraordinary con- 
servation between fly and human sequences. In 
addition, there seems to be a family of TLE genes 
in the human, though not the Drosophila, ge- 
nome. Similarly, we found that the human ge- 
nome contains at least two Notch homologous 
sequences. 
Comparisons among the four isolated human 
TLE genes and the Drosophila counterpart 
showed that they harbor a structural motif impli- 
cated in nuclear-cytoplasmic transport. Referred 
to as the "CcN motif," it consists of a closely 
spaced combination of a nuclear localization se- 
quence and potential phosphorylation sites for 
both casein kinase II and cdc2 kinase. Such a se- 
quence is consistent with the notion that TLEs act 
as nuclear effector molecules. Indeed, immuno- 
cytochemical localizations show that both hu- 
man and Drosophila molecules are predomi- 
nantly nuclear but can also be detected in the 
cytoplasm. 
Surprisingly, comparisons of the known Notch 
homologues, including the two human se- 
quences, reveal the existence of a CcN domain in 
the intracellular part of Notch. We do not yet 
have any evidence regarding the functional signif- 
icance of this motif, which raises the provocative 
possibility that Notch may possess some nuclear 
functions. 
In conclusion, work in the past year extended 
our knowledge regarding the molecular rules un- 
derlying the action of the Notch group genes. 
Our studies — including analyses of expression 
patterns, genetic interactions, and mutant pheno- 
types — reinforce the notion that at least some of 
these genes code for elements of a pleiotropic 
cell interaction mechanism that is involved in 
cell fate choices in many different tissues, in- 
cluding the nervous system. Within those tissues, 
however, cell fate choices can also depend on 
tissue-specific interaction events. Our current 
working hypothesis is that regulative events con- 
trolling cell fate choices throughout develop- 
ment rely on the parallel action of tissue-specific 
mechanisms and on A^ofc^-mediated interactions. 
A^o/^c^ activity, while apparently general, can nev- 
ertheless be modulated by different ligands such 
as Delta or Serrate. 
Expression of Notch protein in the early stages 
of wild-type Drosophila egg chambers (from 
dissected ovaries ) as shown by confocal micros- 
copy. Notch is highly expressed at the apical 
surface of the follicle cells that surround the 
developing egg yolk. 
From Xu, T.. Caron, L.A., Fehon, R.G., and 
Artavanis-Tsakonas, S. 1992. Development 
115:913-922. 
14 
