Abstract
We describe herein the characterization of a major 45-kDa protein from the soluble betaH-crystallin fraction of rhesus monkey (Macaca mulatta) lens. Based on partial peptide sequence, immunoreactivity, and enzymatic activity, this protein has been identified as betaine-homocysteine S-methyltransferase (BHMT: EC 2.1.1.5), an enzyme that catalyzes the methylation of homocysteine using either betaine or thetins as methyl donors. This protein was found to be expressed abundantly in the nuclear region of the monkey lens, reaching approximately 10% of the total nuclear protein, but was barely detectable in the epithelium and cortex regions of the lens. Because the nucleus represents the early embryonic and fetal stages of lens development, we infer that BHMT expression in the lens of the eye is developmentally regulated. By virtue of its high abundance, BHMT can be considered an enzyme crystallin (psi-crystallin). This is the first enzyme crystallin to be found in primate lenses.
Highlights
Crystallins, the major structural proteins of the eye lens, are the primary determinants of the refractive properties of this tissue
Tein was previously reported to be confined to the liver and kidney (6 –9), we report here that it is expressed at very high levels in the central region of the monkey lens, tissue laid down during embryonic and fetal development
By chance observation it was noted that rhesus monkey lens contained a water-soluble protein that migrated on SDS-polyacrylamide gel electrophoresis (PAGE) as a ϳ45-kDa band
Summary
Vol 273, No 46, Issue of November 13, pp. 30669 –30674, 1998 Printed in U.S.A. Betaine-homocysteine Methyltransferase Is a Developmentally Regulated Enzyme Crystallin in Rhesus Monkey Lens*. More than 10 such proteins have been identified and characterized from a variety of vertebrate species [3, 4] It seems that the recruitment of metabolic enzymes as lens crystallins generally occurs through the modification of gene expression without previous gene duplication, very little is known regarding the molecular basis of their high expression in the lens or their selective recruitment to serve as crystallins [2,3,4,5]. In this study we have identified and characterized a prominent protein from Rhesus monkey (Macaca mulatta) lens and found it to be identical to betaine-homocysteine S-methyltransferase (BHMT1: EC 2.1.1.5). Because of its high abundance in the monkey lens nucleus, we consider BHMT to be a developmentally regulated enzyme crystallin, which we have named -crystallin
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