Identification of the molecular networks that facilitated the evolution of multicellular animals from their unicellular ancestors is a fundamental problem in evolutionary cellular biology. Choanoflagellates are recognized as the closest extant nonmetazoan ancestors to animals. These unicellular eukaryotes can adopt a multicellular‐like “rosette” state. Therefore, they are compelling models for the study of early multicellularity. Comparative studies revealed that a number of putative human orthologs are present in choanoflagellate genomes, suggesting that a subset of these genes were necessary for the emergence of multicellularity. However, previous work is largely based on sequence alignments alone, which does not confirm structural nor functional similarity. Here, we focus on the PDZ domain, a peptide‐binding domain which plays critical roles in myriad cellular signaling networks and which underwent a gene family expansion in metazoan lineages. Using a customized sequence similarity search algorithm, we identified 178 PDZ domains in the Monosiga brevicollis proteome. This includes 11 previously unidentified sequences, which we analyzed using Rosetta and homology modeling. To assess conservation of protein structure, we solved high‐resolution crystal structures of representative M. brevicollis PDZ domains that are homologous to human Dlg1 PDZ2, Dlg1 PDZ3, GIPC, and SHANK1 PDZ domains. To assess functional conservation, we calculated binding affinities for mbGIPC, mbSHANK1, mbSNX27, and mbDLG‐3 PDZ domains from M. brevicollis. Overall, we find that peptide selectivity is generally conserved between these two disparate organisms, with one possible exception, mbDLG‐3. Overall, our results provide novel insight into signaling pathways in a choanoflagellate model of primitive multicellularity.

中文翻译：

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Monosiga brevicollis 中 PDZ 域的结构表征和计算分析。

鉴定促进多细胞动物从其单细胞祖先进化的分子网络是进化细胞生物学中的一个基本问题。Choanoflagellates 被认为是现存最接近动物的非后生动物祖先。这些单细胞真核生物可以采用类似多细胞的“玫瑰花结”状态。因此，它们是研究早期多细胞性的引人注目的模型。比较研究表明，许多假定的人类直系同源基因存在于领鞭毛虫基因组中，这表明这些基因的一个子集对于多细胞性的出现是必要的。然而，以前的工作主要基于序列比对，这并不能证实结构或功能的相似性。在这里，我们专注于 PDZ 域，一个肽结合域，在无数的细胞信号网络中起着关键作用，并在后生动物谱系中经历了基因家族的扩展。使用定制的序列相似性搜索算法，我们确定了 178 个 PDZ 域Monosiga brevicollis蛋白质组。这包括 11 个以前未识别的序列，我们使用 Rosetta 和同源建模对其进行了分析。为了评估蛋白质结构的保守性，我们解决了与人 Dlg1 PDZ2、Dlg1 PDZ3、GIPC 和 SHANK1 PDZ 域同源的代表性短颈支原体 PDZ 域的高分辨率晶体结构。为了评估功能保守，我们计算对于mbGIPC，mbSHANK1，mbSNX27，和mbDLG-3 PDZ结构域的结合亲和力从M. brevicollis。总的来说，我们发现这两种不同的生物体之间的肽选择性通常是保守的，一个可能的例外是 mbDLG-3。总体而言，我们的结果为原始多细胞的领鞭毛虫模型中的信号通路提供了新的见解。