Identification of the C-signal, a contact-dependent morphogen coordinating multiple developmental responses in Myxococcus xanthus.

The regulated accumulation of the contact-dependent extracellular C-signal morphogen in the bacterium Myxococcus xanthus ensures the temporal and spatial coordination of multicellular morphogenesis and cellular differentiation during fruiting body formation. Synthesis of the C-signal depends on the csgA gene. The CsgA protein exists in two forms, the full-length 25-kD protein (p25), which is homologous to short-chain alcohol dehydrogenases, and a 17-kD protein (p17). The molecular nature of the C-signal has remained elusive. Here we show that p25 and p17 are associated with the outer membrane and that p17 copurifies with C-signal activity from M. xanthus cells. p17 corresponds to the C-terminal part of p25. A recombinant p17 protein, which lacks the N-terminal coenzyme binding pocket and which fails to bind NAD+ in vitro, has C-signal activity. These data provide evidence that p17 is the active species in C-signaling and that p17 does not act as a short-chain alcohol dehydrogenase to generate the C-signal. We further provide evidence that p17 is synthesized by N-terminal proteolytic processing of p25 by a serine protease. Compared to other bacterial signaling molecules, p17 is unusual with respect to size and cell-surface association. In these regards, C-signal is functionally analogous to eukaryotic signaling proteins.