Schizorhodopsins (SzRs), a new rhodopsin family identified in Asgard archaea, are phylogenetically located at an intermediate position between type-1 microbial rhodopsins and heliorhodopsins. SzRs reportedly work as light-driven inward H⁺ pumps, as xenorhodopsin. Here we report the crystal structure of SzR AM_5_00977 at 2.1 Å resolution. The SzR structure superimposes well on that of bacteriorhodopsin rather than heliorhodopsin, suggesting that SzRs are classified with type-1 rhodopsins. The structure-based mutagenesis study demonstrated that the residues N100 and V103 are essential for color tuning in SzRs. The cytoplasmic parts of transmembrane helices 2, 6, and 7 in SzR are shorter than those in the other microbial rhodopsins. Thus, E81 is located near the cytosol, playing a critical role in the inward H⁺ release. We suggested the H⁺ is not metastably trapped in E81 and released through the water-mediated transport network from the retinal Schiff base to the cytosol. Moreover, most residues on the H⁺ transport pathway are not conserved between SzRs and xenorhodopsins, suggesting that they have entirely different inward H⁺ release mechanisms.

中文翻译：

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五味子视紫红质的晶体结构揭示了质子向内泵浦的机制

Schizorhodopsins（SzRs）是在Asgard古生菌中鉴定出的一种新的视紫红质家族，在系统发育上位于1型微生物视紫红质和heliorhodopsins之间的中间位置。据报道，SzRs可以用作光驱动的内向H ⁺泵，如异视紫红质。在这里，我们报告了2.1Å分辨率下SzR AM_5_00977的晶体结构。SzR结构与细菌视紫红质（而不是heliorhodopsin）重叠得很好，这表明SzRs被分类为1型视紫红质。基于结构的诱变研究表明，残基N100和V103对于SzRs的颜色调节至关重要。SzR中跨膜螺旋2、6和7的胞质部分比其他微生物视紫红质的细胞质部分要短。因此，E81位于胞质溶胶附近，在向内H中起关键作用⁺释放。我们认为H ⁺不能被转移到E81中，而是通过水介导的运输网络从视网膜席夫碱向细胞质释放出来。此外，对H最残⁺转运途径不SzRs和xenorhodopsins之间是保守的，这表明它们具有向内ħ完全不同⁺释放机构。