Determination and Maintenance of Cell Type 
In experiments funded by the National Insti- 
tutes of Health, we have found that Dictyoste- 
lium cells sense whether they are near other 
starving cells by, upon starvation, secreting small 
quantities of a protein that we have named den- 
sity-sensing factor (DSF). Cells are sensitive to 
DSF: above a threshold concentration they will 
express cell-type-specific genes. We have done 
jheoretical diffusion calculations and have 
found, in agreement with our observations, that 
DSF secreted from cells that are quite far from 
other DSF-secreting cells diffuses away so quickly 
that it never reaches the threshold concentration. 
However, at a sufficiently high density of starving 
cells, the DSF concentration will be above the 
threshold value. We have purified DSF protein 
and have found that DSF eventually breaks down 
to small protein fragments that are much more 
effective in activating differentiation. This might 
be a safety mechanism to allow cells that cannot 
find other starving cells to stimulate themselves 
and perhaps form an isolated spore. Interestingly, 
DSF is made and stored in growing cells and is 
only secreted upon starvation. 
We have cloned and sequenced the DNA en- 
coding DSF. This has been used in turn to obtain 
the sequence of amino acids for the DSF protein. 
Computer comparison with data banks of other 
protein sequences indicates that DSF is not 
closely related to any known protein. We have 
used the DSF DNA to generate Dictyostelium 
cells that do not make DSF; these cells do not 
aggregate unless DSF protein is added to their me- 
dium. Dictyostelium cells, which eat bacteria 
that are almost their size, starve asynchronously. 
The behavior of the cells lacking DSF suggests 
that its function is to coordinate development by 
triggering aggregation when most of the cells in a 
given area have starved, as signaled by DSF secre- 
tion, and are ready to form one large fruiting 
body. Finally, we have made antibodies that will 
allow us to examine where DSF is stored in Dic- 
tyostelium cells and to search for similar pro- 
teins in other organisms. 
These two photographs show the expression offasciclin II (brown axon 
bundles) in the developing central nervous system of the grasshopper 
(left) and T)Tosop\\i\2L embryos ( right). Although separated by well over 
300 million years, these two insects continue to express the protein on 
a specific subset of longitudinal axon pathways. The brown staining 
results from HRP immunocytochemistry using anti-fasciclin II anti- 
bodies in both species. 
left photograph from Allan Harrelson and Corey Goodman. Right 
photograph from Gabriele Grenningloh and Corey Goodman. 
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