DataSheet_1_Tenuous transcriptional threshold of human sex determination. II. SRY exploits water-mediated clamp at the edge of ambiguity.pdf

Y-encoded transcription factor SRY initiates male differentiation in therian mammals. This factor contains a high-mobility-group (HMG) box, which mediates sequence-specific DNA binding with sharp DNA bending. A companion article in this issue described sex-reversal mutations at box position 72 (residue 127 in human SRY), invariant as Tyr among mammalian orthologs. Although not contacting DNA, the aromatic ring seals the domain’s minor wing at a solvent-exposed junction with a basic tail. A seeming paradox was posed by the native-like biochemical properties of inherited Swyer variant Y72F: its near-native gene-regulatory activity is consistent with the father’s male development, but at odds with the daughter’s XY female somatic phenotype. Surprisingly, aromatic rings (Y72, F72 or W72) confer higher transcriptional activity than do basic or polar side chains generally observed at solvated DNA interfaces (Arg, Lys, His or Gln). Whereas biophysical studies (time-resolved fluorescence resonance energy transfer and heteronuclear NMR spectroscopy) uncovered only subtle perturbations, dissociation of the Y72F complex was markedly accelerated relative to wild-type. Studies of protein-DNA solvation by molecular-dynamics (MD) simulations of an homologous high-resolution crystal structure (SOX18) suggest that Y72 para-OH anchors a network of water molecules at the tail-DNA interface, perturbed in the variant in association with nonlocal conformational fluctuations. Loss of the Y72 anchor among SRY variants presumably “unclamps” its basic tail, leading to (a) rapid DNA dissociation despite native affinity and (b) attenuated transcriptional activity at the edge of sexual ambiguity. Conservation of Y72 suggests that this water-mediated clamp operates generally among SRY and metazoan SOX domains.

Y染色体编码的转录因子SRY可启动兽亚纲哺乳动物的雄性分化。该因子包含一个高迁移率族（HMG）框，该结构域可介导序列特异性DNA结合并诱导DNA发生剧烈弯曲。本期另一篇论文报道了该框第72位（对应人类SRY的第127位残基）的性反转突变，该位点在哺乳动物同源蛋白中均为酪氨酸（Tyr）。尽管该残基不直接接触DNA，但其芳香环可在结构域的小侧翼与碱性尾段的溶剂暴露连接处形成密封。遗传性Swyer变异体Y72F表现出类似野生型的生化特性，这带来了一个看似矛盾的现象：其接近野生型的基因调控活性本应支持携带该变异体的父亲完成雄性发育，却与携带该变异体的女儿表现出的XY型雌性躯体表型相悖。令人意外的是，相较于通常在溶剂化DNA界面中出现的碱性或极性侧链（精氨酸、赖氨酸、组氨酸或谷氨酰胺），芳香环（Y72、F72或W72）可赋予更高的转录活性。尽管生物物理研究（时间分辨荧光共振能量转移与异核核磁共振波谱法）仅发现了细微的构象扰动，但Y72F变异体复合物的解离速度相较于野生型显著加快。通过对同源高分辨率晶体结构SOX18开展分子动力学（MD）模拟以研究蛋白质-DNA溶剂化特性的结果表明，Y72的对位羟基可在尾段-DNA界面锚定水分子网络，而该锚定在变异体中因伴随非局部构象波动而受到干扰。SRY变异体中Y72锚定的丧失，推测会“松脱”其碱性尾段，进而导致（a）尽管亲和力接近野生型但DNA解离速度加快，以及（b）在性别分化临界区间内转录活性减弱。Y72位点的保守性提示，这种水介导的锚定机制普遍存在于SRY以及后生动物的SOX结构域中。