Two roles for mu-crystallin: a lens structural protein in diurnal marsupials and a possible enzyme in mammalian retinas.

PURPOSE: 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.
METHODS: 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.
RESULTS: 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.
CONCLUSIONS: 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.