Segovia L, Horwitz J, Gasser R, Wistow G
Section on Molecular Structure and Function, National Eye Institute, National Institutes of Health, Bethesda, MD, USA.
Mol Vis. 1997 Sep 9;3:9.
micro-Crystallin is a major taxon-specific lens protein in some marsupials. Like other taxon-specific crystallins, it probably has another, non-crystallin role. Here we examine the distribution of mu-crystallin among species and its localization in the eye in placental mammals. We also compare its sequence and ligand binding characteristics with those of known enzymes.
An antibody (Mup1) was raised against a conserved 21 residue peptide of tammar wallaby mu-crystallin. This was used to detect mu-crystallin immunoreactivity in lens extracts of several species and in the tissues of rat and bovine eyes. PCR methods were used to complete the cDNA sequence of human mu-crystallin. The ability of kangaroo mu-crystallin to bind enzymatic cofactors was tested by blue-sepharose chromatography.
Using Mup1, abundant mu-crystallin was observed in soluble whole lens extracts of diurnal Australian marsupials. Although abundant micro-crystallin was not detectable in whole lens of nocturnal marsupials, other mammals or a bird, lower levels of immunoreactivity were detectable in lens epithelium, retinal pigment epithelium and, particularly, retina of bovine eye. In rat eye the highest levels of Mup1 reactivity were found in retinal photoreceptors. Sequence comparisons of human and kangaroo mu-crystallin reveal a superfamily relationship with enzymes of glutamate and ornithine metabolism. Co-factor binding studies indicate that mu-crystallin, like related glutamyl-tRNA reductases, binds NADPH.
These results suggest that mu-crystallin is a normal component of retina and other tissues which underwent gene recruitment to gain an additional structural role in the lens during the evolution of diurnal marsupial species. mu-crystallin may be an enzyme, possibly of amino acid metabolism, with particular importance for photoreceptors.
微晶体蛋白是一些有袋动物中主要的分类群特异性晶状体蛋白。与其他分类群特异性晶状体蛋白一样，它可能具有另一种非晶状体蛋白的功能。在此，我们研究了微晶体蛋白在物种间的分布及其在胎盘哺乳动物眼中的定位。我们还将其序列和配体结合特性与已知酶进行了比较。
制备了一种针对短尾矮袋鼠微晶体蛋白保守的21个氨基酸肽段的抗体（Mup1）。用其检测几种物种晶状体提取物以及大鼠和牛眼组织中的微晶体蛋白免疫反应性。采用PCR方法完成人微晶体蛋白的cDNA序列。通过蓝琼脂糖层析检测袋鼠微晶体蛋白结合酶辅因子的能力。
使用Mup1抗体，在澳大利亚昼行性有袋动物的可溶性全晶状体提取物中观察到大量微晶体蛋白。虽然在夜行性有袋动物、其他哺乳动物或鸟类的全晶状体中未检测到大量微晶体蛋白，但在晶状体上皮、视网膜色素上皮，尤其是牛眼视网膜中可检测到较低水平的免疫反应性。在大鼠眼中，Mup1反应性最高的部位是视网膜光感受器。人和袋鼠微晶体蛋白的序列比较显示与谷氨酸和鸟氨酸代谢酶存在超家族关系。辅因子结合研究表明，微晶体蛋白与相关的谷氨酰-tRNA还原酶一样，能结合NADPH。
这些结果表明，微晶体蛋白是视网膜和其他组织的正常成分，在昼行性有袋动物物种进化过程中，该基因被招募以在晶状体中获得额外的结构作用。微晶体蛋白可能是一种酶，可能参与氨基酸代谢，对光感受器尤为重要。
