Summary auto-generated
This study investigates the molecular basis of intercellular signaling between forespore and mother-cell compartments during Bacillus subtilis sporulation. The researchers used site-directed mutagenesis to analyze the SpoIIGA receptor and SpoIIR signal proteins that mediate σE activation in the mother-cell. They mapped SpoIIGA's membrane topology and identified that a conserved asparagine residue at position 6 (D6) in the N-terminal extracellular region is essential for receptor function, while the major extracellular loop between membrane domains IV and V is dispensable. In contrast, mutations blocking SpoIIR protein release from the membrane—while preventing cleavage of the signal peptide—did not impair signaling, indicating that membrane translocation, not release, is required for SpoIIR activity. The results suggest that the forespore-mother-cell signaling pathway involves direct interactions at the membrane level, and do not exclude the possibility that additional unidentified molecules participate in this developmental communication system.
Key findings
- The N-terminal aspartate at position 6 (D6) of SpoIIGA is critical for receptor function in forespore-mother-cell signaling
- The large extracellular loop between SpoIIGA membrane domains IV and V is dispensable for sensing or transducing the activation signal
- SpoIIR protein must be translocated across the membrane but does not need to be released/cleaved from the membrane to activate SpoIIGA
- Additional uncharacterized molecules may participate in the mother-cell-specific developmental program during early sporulation
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Abstract
Summary: Intercellular communication is a crucial phenomenon during spore development in Bacillus subtilis. It couples the establishment of a compartment-specific genetic program to the transcriptional activity of a σ factor in the other compartment. It also keeps σ factor activation in register with the morphological process. This study used directed mutagenesis to analyse the pathway that couples σE activation in the mother-cell to activation of σF in the forespore following asymmetric septation. Targets for mutagenesis in SpollGA (the receptor) were chosen based on the predicted topology of the protein when associated with the cell membrane. The results showed that a residue near the N terminus (D6), predicted to be exposed outside the cell, is required for receptor activity, whereas the major extracellular loop (between membrane domains IV and V) is dispensable for function. In contrast, mutations in SpollR (the signal) that partially blocked protein release (but not membrane translocation) had no effect on signal transduction. These results do not rule out the possibility that uncharacterized molecules intervene in the signalling pathway that establishes the mother-cell-specific developmental program during the early stage of sporulation.