Yamniuk Aaron P, Burling Hans, Vogel Hans J
Structural Biology Research Group, Department of Biological Sciences, University of Calgary, Calgary, AB, Canada.
Mol Immunol. 2009 Jul;46(11-12):2395-402. doi: 10.1016/j.molimm.2009.04.024. Epub 2009 May 23.
The immunoregulatory proteins osteopontin (OPN) and lactoferrin (LF) are both highly abundant in milk, with a conserved ratio between different mammalian species, suggesting that the role of each protein in infant development may be linked. In this study we used isothermal titration calorimetry and differential scanning calorimetry experiments to demonstrate that LF and OPN interact with each other through a complex mechanism involving multiple cationic LF molecules binding to a single anionic molecule of OPN. At least two classes of thermodynamically distinct LF binding sites were identified, with the higher affinity interactions (dissociation constants 10(-6)M) being in the biologically relevant range. Ca(2+) binding to OPN, or Fe(3+) binding to LF, influenced the enthalpy and entropy of binding, but had little effect on the overall binding affinity. Considering that the regions of electrostatic complementarity between OPN and LF mediate the numerous biological functions of each protein, we suggest that OPN may act as a carrier protein for LF in milk, and modulate the potent antimicrobial and immunostimulatory activities of the LF protein.
免疫调节蛋白骨桥蛋白（OPN）和乳铁蛋白（LF）在乳汁中含量都非常丰富，在不同哺乳动物物种之间存在保守的比例，这表明每种蛋白质在婴儿发育中的作用可能相互关联。在本研究中，我们使用等温滴定量热法和差示扫描量热法实验证明，LF和OPN通过一种复杂机制相互作用，该机制涉及多个阳离子LF分子与单个阴离子OPN分子结合。至少鉴定出两类热力学上不同的LF结合位点，其中亲和力较高的相互作用（解离常数为10^(-6)M）处于生物学相关范围内。Ca(2+)与OPN结合，或Fe(3+)与LF结合，会影响结合的焓和熵，但对整体结合亲和力影响不大。鉴于OPN和LF之间的静电互补区域介导了每种蛋白质的众多生物学功能，我们认为OPN可能在乳汁中作为LF的载体蛋白，并调节LF蛋白强大的抗菌和免疫刺激活性。
