MBE Advance Access originally published online on April 25, 2003
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Mol. Biol. Evol. 20(6):994-998. 2003
DOI: 10.1093/molbev/msg116
© 2003 by the Society for Molecular Biology and Evolution. ISSN: 0737-4038
Lactate Dehydrogenase A as a Highly Abundant Eye Lens Protein in Platypus (Ornithorhynchus anatinus): Upsilon (
)-Crystallin
* Department of Biochemistry, University of Nijmegen, Nijmegen, The Netherlands
Leiden University Medical Center, Leiden, The Netherlands
School of Aquaculture, University of Tasmania, Hobart, Australia
Vertebrate eye lenses mostly contain two abundant types of proteins, the 
-crystallins and the ß/
-crystallins. In addition, certain housekeeping enzymes are highly expressed as crystallins in various taxa. We now observed an unusual approximately 41-kd protein that makes up 16% to 18% of the total protein in the platypus eye lens. Its cDNA sequence was determined, which identified the protein as muscle-type lactate dehydrogenase A (LDH-A). It is the first observation of LDH-A as a crystallin, and we designate it upsilon (
)-crystallin. Interestingly, the related heart-type LDH-B occurs as an abundant lens protein, known as 
-crystallin, in many birds and crocodiles. Thus, two members of the ldh gene family have independently been recruited as crystallins in different higher vertebrate lineages, suggesting that they are particularly suited for this purpose in terms of gene regulatory or protein structural properties. To establish whether platypus LDH-A/-crystallin has been under different selective constraints as compared with other vertebrate LDH-A sequences, we reconstructed the vertebrate ldh-a gene phylogeny. No conspicuous rate deviations or amino acid replacements were observed.
Key Words: platypus • Ornithorhynchus anatinus • -crystallin • taxon-specific crystallin • lactate dehydrogenase • gene sharing • mammalian phylogeny