Lotte Sogaard-Andersen
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Bacterial development and differentiation
Intercellular signaling in fruiting body formation
The C-signal is one of two functionally and biochemically characterized intercellular signals required for fruiting body formation. The C-signal is the intercellular signal that induces and coordinates aggregation of cells into nascent fruiting bodies and sporulation of cells that have accumulated inside the fruiting bodies. We have previously shown that the C-signal is an unusual signaling molecule: C-signal synthesis depends on the csgA gene that encodes the full-length 25 kDa CsgA protein (p25), which is anchored in the outer membrane. p25 accumulates in vegetative cells as well as in starving cells. During starvation p25 is cleaved to a 17 kDa protein, which is also anchored in the outer membrane and exposed on the cell surface. This 17 kDa protein (p17) is the actual C-signal. Thus, unlike other intercellular signaling molecules identified in bacteria, the C-signal is not a small diffusible molecule rather it is a relatively large cell-surface associated protein and signal transmission depends on direct cell-cell contacts.
To address how p25 cleavage and, thus, p17 accumulation, is restricted to starving cells, we have focused on the identification of the protease (PopC) that cleaves p25. Using a four-tiered strategy involving bioinformatics, the biochemical characteristics of proteases, genome-wide transcriptional profiling of M. xanthus, and reverse genetics to identify popC candidate genes, we identified the MXAN0206 gene as the gene that encodes PopC. In vegetative cells PopC is localized to the cytoplasm and not secreted whereas in starving cells PopC is secreted. According to our current model, p25 and PopC accumulate in vegetative cells; however, p25 is not cleaved because p25 and PopC are localized to different cell compartments. In response to starvation, PopC secretion is licensed and PopC is now in the same cell compartment as p25 and, consequently, p25 cleavage occurs. Thus, the mechanism underlying regulated proteolysis of p25 depends on regulated secretion of PopC in response to starvation. Our current research focuses on understanding how PopC secretion is regulated.
Regulated proteolysis of p25 by regulated secretion of PopC in M. xanthus