Small proteins maintain the fidelity of spore envelope assembly through regulated cell death during sporulation in Bacillus subtilis

[摘要] Under conditions of nutrient deprivation, the bacterium Bacillus subtilis undergoes a relatively simple, alternative developmental pathway called sporulation that results in the production of a largely metabolically dormant cell type called a spore. The spore is encased in two concentric shells, the coat and the cortex, which are important for maintaining the spore’s dormancy and protecting it from various environmental insults. Despite being spatially separated by a membrane layer, the assembly of these two structures, which form the spore envelope, is linked and orchestrated by two coat proteins, SpoVM and SpoIVA.SpoVM and SpoIVA are the only coat proteins known to be involved in cortex assembly indicating that they play a unique role in coordinating the assembly of the coat and cortex. While the roles of SpoVM and SpoIVA in coat assembly are well-understood, their roles in cortex assembly and the mechanisms that regulate the coordinated assembly of coat and cortex remain a mystery. Using classical genetics to select for spontaneous suppressors of cortex-deficient mutants, we discovered a sporulation checkpoint where a previously un-annotated gene encodes a small 37-amino-acid protein, CmpA. We found that CmpA is degraded in wild type cells, but persists in a cortex-deficient mutant and propose a model in which the degradation of CmpA signals proper initiation of coat assembly.However, when a cell is unable to properly initiate coat assembly, CmpA persists and functions as an adaptor to deliver the morphogenetic protein SpoIVA to the AAA+ protease ClpXP for degradation. Mutations affecting complex formation between CmpA, SpoIVA and ClpX prevented degradation of SpoIVA indicating that they were sufficient to bypass the checkpoint and sporulation was able to proceed. The degradation of SpoIVA initiates an apoptotic-like event which leads to cell lysis. We believe this to be a mechanism to selectively remove unfit cells that are unable to properly assemble the spore envelope. Left unchecked, mutations in sporulating genes accumulate readily and ultimately lead to loss of the sporulation program. Thus, the CmpA-dependent regulated cell death pathway actively selects against defective cells in order to maintain the integrity of the sporulation program.

[发布机构] Johns Hopkins University

[效力级别] bacillus

[关键词] sporulation;bacillus;ClpXP;CmpA;checkpoint;bacteria;Biology