Ahrendt Steven R, Medina Edgar Mauricio, Chang Chia-En A, Stajich Jason E
Department of Plant Pathology & Microbiology, University of California, Riverside, CA, USA.
Institute for Integrative Genome Biology, University of California, Riverside, CA, USA.
PeerJ. 2017 Apr 27;5:e3206. doi: 10.7717/peerj.3206. eCollection 2017.
Opsin proteins are seven transmembrane receptor proteins which detect light. Opsins can be classified into two types and share little sequence identity: type 1, typically found in bacteria, and type 2, primarily characterized in metazoa. The type 2 opsins (Rhodopsins) are a subfamily of G-protein coupled receptors (GPCRs), a large and diverse class of seven transmembrane proteins and are generally restricted to metazoan lineages. Fungi use light receptors including opsins to sense the environment and transduce signals for developmental or metabolic changes. Opsins characterized in the Dikarya (Ascomycetes and Basidiomycetes) are of the type 1 bacteriorhodopsin family but the early diverging fungal lineages have not been as well surveyed. We identified by sequence similarity a rhodopsin-like GPCR in genomes of early diverging chytrids and examined the structural characteristics of this protein to assess its likelihood to be homologous to animal rhodopsins and bind similar chromophores.
We used template-based structure modeling, automated ligand docking, and molecular modeling to assess the structural and binding properties of an identified opsin-like protein found in , a unicellular, flagellated species belonging to Chytridiomycota, one of the earliest diverging fungal lineages. We tested if the sequence and inferred structure were consistent with a solved crystal structure of a type 2 rhodopsin from the squid .
Our results indicate that the opsin has structural characteristics consistent with functional animal type 2 rhodopsins and is capable of maintaining a stable structure when associated with the retinaldehyde chromophore, specifically the 9-retinal isomer. Together, these results support further the homology of opsins to animal type 2 rhodopsins.
This represents the first test of structure/function relationship of a type 2 rhodopsin identified in early branching fungal lineages, and provides a foundation for future work exploring pathways and components of photoreception in early fungi.
视蛋白是检测光的七跨膜受体蛋白。视蛋白可分为两类，序列同源性很低：第1类，通常存在于细菌中；第2类，主要存在于后生动物中。第2类视蛋白（视紫红质）是G蛋白偶联受体（GPCRs）的一个亚家族，GPCRs是一大类多样的七跨膜蛋白，一般局限于后生动物谱系。真菌利用包括视蛋白在内的光受体感知环境并转导信号以实现发育或代谢变化。在双核菌纲（子囊菌门和担子菌门）中鉴定出的视蛋白属于第1类细菌视紫红质家族，但早期分化的真菌谱系尚未得到充分研究。我们通过序列相似性在早期分化的壶菌基因组中鉴定出一种视紫红质样GPCR，并研究了该蛋白结构特征，以评估其与动物视紫红质同源并结合相似发色团的可能性。
我们使用基于模板的结构建模、自动配体对接和分子建模来评估在 中发现的一种已鉴定的视蛋白样蛋白的结构和结合特性， 是属于壶菌门的单细胞、有鞭毛的物种，壶菌门是最早分化的真菌谱系之一。我们测试了该序列和推断结构是否与来自鱿鱼的第2类视紫红质的已解析晶体结构一致。
我们的结果表明， 视蛋白具有与功能性动物第2类视紫红质一致的结构特征，并且在与视黄醛发色团（特别是9-视黄醛异构体）结合时能够保持稳定结构。这些结果共同进一步支持了 视蛋白与动物第2类视紫红质的同源性。
这是对在早期分支真菌谱系中鉴定出的第2类视紫红质的结构/功能关系的首次测试，并为未来探索早期真菌光感受途径和成分的工作奠定了基础。
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