Data_Sheet_1_Phylogenetic Distribution and Diversity of Bacterial Pseudo-Orthocaspases Underline Their Putative Role in Photosynthesis.zip

Orthocaspases are prokaryotic caspase homologs – proteases, which cleave their substrates after positively charged residues using a conserved histidine – cysteine (HC) dyad situated in a catalytic p20 domain. However, in orthocaspases pseudo-variants have been identified, which instead of the catalytic HC residues contain tyrosine and serine, respectively. The presence and distribution of these presumably proteolytically inactive p20-containing enzymes has until now escaped attention. We have performed a detailed analysis of orthocaspases in all available prokaryotic genomes, focusing on pseudo-orthocaspases. Surprisingly we identified type I metacaspase homologs in filamentous cyanobacteria. While genes encoding pseudo-orthocaspases seem to be absent in Archaea, our results show conservation of these genes in organisms performing either anoxygenic photosynthesis (orders Rhizobiales, Rhodobacterales, and Rhodospirillales in Alphaproteobacteria) or oxygenic photosynthesis (all sequenced cyanobacteria, except Gloeobacter, Prochlorococcus, and Cyanobium). Contrary to earlier reports, we were able to detect pseudo-orthocaspases in all sequenced strains of the unicellular cyanobacteria Synechococcus and Synechocystis. In silico comparisons of the primary as well as tertiary structures of pseudo-p20 domains with their presumably proteolytically active homologs suggest that differences in their amino acid sequences have no influence on the overall structures. Mutations therefore affect most likely only the proteolytic activity. Our data provide an insight into diversification of pseudo-orthocaspases in Prokaryotes, their taxa-specific distribution, and allow suggestions on their taxa-specific function.

直向半胱天冬酶（orthocaspase）是原核来源的半胱天冬酶同源物，属于一类蛋白酶，可通过位于催化性p20结构域中的保守组氨酸-半胱氨酸（HC）二联体，在底物的带正电荷残基位点处完成切割反应。然而，研究人员已在直向半胱天冬酶中鉴定出假变体：这类变体的催化HC二联体分别被酪氨酸与丝氨酸取代。此前，这类推测无蛋白水解活性的、含p20结构域的酶的存在与分布规律尚未受到研究关注。本研究对所有已公开的原核基因组中的直向半胱天冬酶展开了详细分析，重点聚焦于假直向半胱天冬酶。令人意外的是，我们在丝状蓝细菌中鉴定出了I型半胱天冬酶（metacaspase）同源物。尽管古菌中似乎不存在编码假直向半胱天冬酶的基因，但我们的研究结果显示，这类基因在进行不产氧光合作用的生物（α-变形菌门中的根瘤菌目、红杆菌目及红螺菌目）以及产氧光合作用的生物（除黏杆蓝细菌（Gloeobacter）、原绿球藻（Prochlorococcus）和蓝杆藻（Cyanobium）外的所有已测序蓝细菌）中均具有保守性。与此前的研究报道相反，我们在单细胞蓝细菌聚球藻（Synechococcus）和集胞藻（Synechocystis）的所有已测序菌株中均检测到了假直向半胱天冬酶。通过计算机模拟对假p20结构域与其推测具有蛋白水解活性的同源物的一级及三级结构进行比对后发现，二者的氨基酸序列差异并未对整体结构造成影响，因此这类突变大概率仅影响其蛋白水解活性。本研究的数据为探究原核生物中假直向半胱天冬酶的多样性、类群特异性分布规律提供了新的见解，并可为揭示其类群特异性功能提供参考。