Phylogenetic Analysis of Glycerol 3-Phosphate Acyltransferases in Opisthokonts Reveals Unexpected Ancestral Complexity and Novel Modern Biosynthetic Components

Glycerolipid synthesis represents a central metabolic process of all forms of life. In the last decade multiple genes coding for enzymes responsible for the first step of the pathway, catalyzed by glycerol 3-phosphate acyltransferase (GPAT), have been described, and characterized primarily in model organisms like Saccharomyces cerevisiae and mice. Notoriously, the fungal enzymes share low sequence identity with their known animal counterparts, and the nature of their homology is unclear. Furthermore, two mitochondrial GPAT isoforms have been described in animal cells, while no such enzymes have been identified in Fungi. In order to determine if the yeast and mammalian GPATs are representative of the set of enzymes present in their respective groups, and to test the hypothesis that metazoan orthologues are indeed absent from the fungal clade, a comparative genomic and phylogenetic analysis was performed including organisms spanning the breadth of the Opisthokonta supergroup. Surprisingly, our study unveiled the presence of ‘fungal’ orthologs in the basal taxa of the holozoa and ‘animal’ orthologues in the basal holomycetes. This includes a novel clade of fungal homologues, with putative peroxisomal targeting signals, of the mitochondrial/peroxisomal acyltransferases in Metazoa, thus potentially representing an undescribed metabolic capacity in the Fungi. The overall distribution of GPAT homologues is suggestive of high relative complexity in the ancestors of the opisthokont clade, followed by loss and sculpting of the complement in the descendent lineages. Divergence from a general versatile metabolic model, present in ancestrally deduced GPAT complements, points to distinctive contributions of each GPAT isoform to lipid metabolism and homeostasis in contemporary organisms like humans and their fungal pathogens.甘油酯（Glycerolipid）合成是所有生命形式的核心代谢过程。近十年来，多个编码该通路第一步反应酶（该反应由甘油3-磷酸酰基转移酶（glycerol 3-phosphate acyltransferase, GPAT）催化）的基因已被报道，并主要在酿酒酵母（Saccharomyces cerevisiae）和小鼠等模式生物中完成了功能表征。值得注意的是，真菌来源的此类酶与已报道的动物同源酶序列一致性较低，二者的同源性本质仍未阐明。此外，动物细胞中已发现两种线粒体GPAT同工型（isoform），但真菌界中尚未鉴定到此类酶。为验证酵母与哺乳动物GPAT是否分别代表其所属类群中的酶系，并检验“真菌演化支（clade）中确实不存在后生动物直向同源物（orthologue）”这一假说，本研究开展了比较基因组学与系统发育分析，分析对象覆盖了后鞭毛生物总界（Opisthokonta）的所有主要类群。令人意外的是，本研究发现：全动物类群（holozoa）的基干类群中存在“真菌来源”的直向同源物，而基干全真菌类群（holomycetes）中则存在“动物来源”的直向同源物。其中包括一类新发现的真菌同源物演化支：该类同源物带有预测的过氧化物酶体靶向信号（peroxisomal targeting signals），对应后生动物（Metazoa）中的线粒体/过氧化物酶体酰基转移酶，这可能意味着真菌界中存在尚未被报道的代谢能力。GPAT同源物的整体分布模式表明，后鞭毛生物演化支的祖先拥有相对复杂的酶系，后续的演化支中则经历了酶系的丢失与重塑。从祖先推导得到的通用多功能GPAT酶系模型发生的分化，表明不同GPAT同工型在现代生物（如人类及其真菌病原体）的脂质代谢与稳态维持中发挥着独特作用。