Department of Biochemistry, University of Vermont, Burlington, Vermont 05405, USA.
Protein Sci. 2010 Sep;19(9):1616-26. doi: 10.1002/pro.439.
The original signature of the transferrin (TF) family of proteins was the ability to bind ferric iron with high affinity in the cleft of each of two homologous lobes. However, in recent years, new family members that do not bind iron have been discovered. One new member is the inhibitor of carbonic anhydrase (ICA), which as its name indicates, binds to and strongly inhibits certain isoforms of carbonic anhydrase. Recently, mouse ICA has been expressed as a recombinant protein in a mammalian cell system. Here, we describe the 2.4 Å structure of mouse ICA from a pseudomerohedral twinned crystal. As predicted, the structure is bilobal, comprised of two α-β domains per lobe typical of the other family members. As with all but insect TFs, the structure includes the unusual reverse γ-turn in each lobe. The structure is consistent with the fact that introduction of two mutations in the N-lobe of murine ICA (mICA) (W124R and S188Y) allowed it to bind iron with high affinity. Unexpectedly, both lobes of the mICA were found in the closed conformation usually associated with presence of iron in the cleft, and making the structure most similar to diferric pig TF. Two new ICA family members (guinea pig and horse) were identified from genomic sequences and used in evolutionary comparisons. Additionally, a comparison of selection pressure (dN/dS) on functional residues reveals some interesting insights into the evolution of the TF family including that the N-lobe of lactoferrin may be in the process of eliminating its iron binding function.
转铁蛋白（TF）家族蛋白的原始特征是能够在两个同源结构域的每个结构域的裂缝中高亲和力地结合三价铁。然而，近年来，发现了一些不结合铁的新家族成员。一个新成员是碳酸酐酶抑制剂（ICA），如其名所示，它与某些同工型的碳酸酐酶结合并强烈抑制这些同工型。最近，在哺乳动物细胞系统中表达了鼠 ICA 的重组蛋白。在这里，我们描述了来自拟准二倍体孪晶的鼠 ICA 的 2.4 Å 结构。如预测的那样，该结构是双叶的，每个结构域由两个α-β结构域组成，这是其他家族成员的典型特征。与除昆虫 TF 以外的所有 TF 一样，该结构包括每个结构域中不寻常的反向γ-转角。该结构与以下事实一致，即鼠 ICA（mICA）的 N 结构域中的两个突变（W124R 和 S188Y）的引入使其能够高亲和力地结合铁。出乎意料的是，mICA 的两个结构域都处于通常与铁在裂缝中存在相关的闭合构象，并且使结构与二价猪 TF 最相似。从基因组序列中鉴定出了两个新的 ICA 家族成员（豚鼠和马），并用于进化比较。此外，对功能残基上的选择压力（dN/dS）的比较揭示了 TF 家族进化的一些有趣见解，包括乳铁蛋白的 N 结构域可能正在消除其铁结合功能。
