Dictyostelium discoideum spends most of its life as vegetative amoebae, but, when food becomes limiting, the amoebae stop dividing, aggregate and develop into a mass of spores supported by a stalk. Unexpectedly, Brock and Gomer have now found that growing Dictyostelium cells secrete a protein (called AprA, for autocrine proliferation repressor) that represses their proliferation (see p. ). They show that aprA-null cells proliferate faster than wild-type cells, and that purified AprA slows the proliferation of both wild-type and aprA-null cells. However, upon starvation, aprA-null cells make spores less efficiently than wild-type cells do. Overall, these results indicate that there is an evolutionary advantage to slowing proliferation when Dictyostelium cells get crowded. In addition, the researchers suggest that AprA may be part of a Dictyostelium chalone–in metazoa,chalones are secreted autocrine factors thought to control organ size.
