Kevorkian Yuzo, Shirley David J, Shen Aimee
Department of Microbiology and Molecular Genetics, University of Vermont, Burlington, VT, 05405, USA.
Department of Microbiology and Molecular Genetics, University of Vermont, Burlington, VT, 05405, USA.
Biochimie. 2016 Mar;122:243-54. doi: 10.1016/j.biochi.2015.07.023. Epub 2015 Jul 29.
Clostridium difficile is a spore-forming obligate anaerobe that is a leading cause of healthcare-associated infections. C. difficile infections begin when its metabolically dormant spores germinate in the gut of susceptible individuals. Binding of bile salt germinants to the Csp family pseudoprotease CspC triggers a proteolytic signaling cascade consisting of the Csp family protease CspB and the cortex hydrolase SleC. Conserved across many of the Clostridia, Csp proteases are subtilisin-like serine proteases that activate pro-SleC by cleaving off its inhibitory pro-peptide. Active SleC degrades the protective cortex layer, allowing spores to resume metabolism and growth. This signaling pathway, however, is differentially regulated in C. difficile, since CspC functions both as a germinant receptor and regulator of CspB activity. CspB is also produced as a fusion to a catalytically inactive CspA domain that subsequently undergoes interdomain processing during spore formation. In this study, we investigated the role of the CspA pseudoprotease domain in regulating C. difficile spore germination. Mutational analyses revealed that the CspA domain controls CspC germinant receptor levels in mature spores and is required for optimal spore germination, particularly when CspA is fused to the CspB protease. During spore formation, the YabG protease separates these domains, although YabG itself is dispensable for germination. Bioinformatic analyses of Csp family members suggest that the CspC-regulated signaling pathway characterized in C. difficile is conserved in related Peptostreptococcaceae family members but not in the Clostridiaceae or Lachnospiraceae. Our results indicate that pseudoproteases play critical roles in regulating C. difficile spore germination and highlight that diverse mechanisms control spore germination in the Clostridia.
艰难梭菌是一种形成孢子的专性厌氧菌,是医疗保健相关感染的主要原因。当艰难梭菌代谢休眠的孢子在易感个体的肠道中萌发时,艰难梭菌感染就会开始。胆盐萌发剂与Csp家族假蛋白酶CspC结合会触发一个蛋白水解信号级联反应,该反应由Csp家族蛋白酶CspB和皮层水解酶SleC组成。Csp蛋白酶在许多梭菌属中都很保守,是枯草杆菌蛋白酶样丝氨酸蛋白酶,通过切割前SleC的抑制性前肽来激活它。活性SleC降解保护性皮层,使孢子恢复代谢和生长。然而,这条信号通路在艰难梭菌中受到不同的调节,因为CspC既作为萌发剂受体又作为CspB活性的调节剂。CspB也作为与催化无活性的CspA结构域的融合蛋白产生,随后在孢子形成过程中进行结构域间加工。在这项研究中,我们研究了CspA假蛋白酶结构域在调节艰难梭菌孢子萌发中的作用。突变分析表明,CspA结构域控制成熟孢子中CspC萌发剂受体的水平,是最佳孢子萌发所必需的,特别是当CspA与CspB蛋白酶融合时。在孢子形成过程中,YabG蛋白酶将这些结构域分开,尽管YabG本身对于萌发是可有可无的。对Csp家族成员的生物信息学分析表明,艰难梭菌中以CspC为特征的信号通路在相关的消化链球菌科家族成员中是保守的,但在梭菌科或毛螺菌科中则不然。我们的结果表明,假蛋白酶在调节艰难梭菌孢子萌发中起关键作用,并突出了多种机制控制梭菌属孢子萌发。