Department of Biological Sciences, University of Manitoba, Winnipeg, Canada.
Department of Zoology, University of British Columbia, Vancouver, Canada.
Elife. 2023 Jun 1;12:e85414. doi: 10.7554/eLife.85414.
The extinct Steller's sea cow (; †1768) was a whale-sized marine mammal that manifested profound morphological specializations to exploit the harsh coastal climate of the North Pacific. Yet despite first-hand accounts of their biology, little is known regarding the physiological adjustments underlying their evolution to this environment. Here, the adult-expressed hemoglobin (Hb; αβ/δ) of this sirenian is shown to harbor a fixed amino acid replacement at an otherwise invariant position (β/δ82Lys→Asn) that alters multiple aspects of Hb function. First, our functional characterization of recombinant sirenian Hb proteins demonstrates that the Hb-O affinity of this sub-Arctic species was less affected by temperature than those of living (sub)tropical sea cows. This phenotype presumably safeguarded O delivery to cool peripheral tissues and largely arises from a reduced intrinsic temperature sensitivity of the protein. Additional experiments on β/δ82Asn→Lys mutant Hb further reveal this exchange renders Steller's sea cow Hb unresponsive to the potent intraerythrocytic allosteric effector 2,3-diphosphoglycerate, a radical modification that is the first documented example of this phenotype among mammals. Notably, β/δ82Lys→Asn moreover underlies the secondary evolution of a reduced blood-O affinity phenotype that would have promoted heightened tissue and maternal/fetal O delivery. This conclusion is bolstered by analyses of two Steller's sea cow prenatal Hb proteins (Hb Gower I; ζε and HbF; αγ) that suggest an exclusive embryonic stage expression pattern, and reveal uncommon replacements in HbF (γ38Thr→Ile and γ101Glu→Asp) that increased Hb-O affinity relative to dugong HbF. Finally, the β/δ82Lys→Asn replacement of the adult/fetal protein is shown to increase protein solubility, which may have elevated red blood cell Hb content within both the adult and fetal circulations and contributed to meeting the elevated metabolic (thermoregulatory) requirements and fetal growth rates associated with this species cold adaptation.
已灭绝的斯特勒海牛（†1768）是一种体型如鲸的海洋哺乳动物，其形态发生了深刻的特化，以适应北太平洋恶劣的沿海气候。然而，尽管有关于它们生物学的第一手资料，但对于它们进化到这种环境的生理适应知之甚少。在这里，展示了这种海牛的成人表达血红蛋白（Hb；αβ/δ）在一个不变位置（β/δ82Lys→Asn）上存在固定的氨基酸替换，该替换改变了 Hb 功能的多个方面。首先，我们对重组海牛 Hb 蛋白的功能表征表明，这种亚北极物种的 Hb-O 亲和力受温度的影响小于生活在（亚热带）热带的海牛。这种表型可能保护了向凉爽的外周组织输送 O，并主要源自该蛋白内在温度敏感性的降低。对β/δ82Asn→Lys 突变 Hb 的进一步实验进一步表明，这种交换使斯特勒海牛 Hb 对强效红细胞内变构效应物 2,3-二磷酸甘油酸无反应，这种激进的修饰是哺乳动物中首次记录到的这种表型的例子。值得注意的是，β/δ82Lys→Asn 还导致了血液-O 亲和力降低的表型的二次进化，这将促进组织和母体/胎儿 O 的输送。这一结论得到了对两种斯特勒海牛产前 Hb 蛋白（Hb Gower I；ζε和 HbF； αγ）的分析的支持，这些分析表明它们仅在胚胎阶段表达，并揭示了 HbF 中不常见的替换（γ38Thr→Ile 和γ101Glu→Asp），这增加了 Hb-O 亲和力，与儒艮 HbF 相比。最后，证明了成人/胎儿蛋白中的β/δ82Lys→Asn 替换增加了蛋白质的溶解度，这可能增加了成人和胎儿循环中的红细胞 Hb 含量，并有助于满足与该物种冷适应相关的代谢（体温调节）需求和胎儿生长速度的提高。
