Relationship between proteins encoded by three human gamma-crystallin genes and distinct polypeptides in the eye lens.

Abstract

Although individual -y-crystallins from the human eye lens have not been successfully purffied and sequenced, most of the genes coding for these lens-specific structural proteins have been cloned and characterized. To investigate the relationship between these genes and the y-crystallins of the human lens, we made use of mouse cell lines which contain stably integrated copies of the coding sequences for three of the human -y-crystallin genes coupled to the human metallothionein IIA promoter. The proteins produced by these hybrid genes in cell culture were detected immunologically and compared by physical characteristics with the -y-crystallins from the human lens. The protein encoded by the G3 gene showed properties identical to those of the 21,000-molecular-weight y-crystallin from 11-month-old lens. The protein isolated from the cells expressing the G4 gene was similar to a 19,000-molecular-weight lens y-crystallin, while gene G5 encodes a highly basic -y-crystallin which may be synthesized in only limited amounts in the human lens. These correlations provide a basis for future investigations on the relationship between putative mutations in human -y-crystallin genes and altered proteins in hereditary lens cataracts.The human lens is a biconvex, avascular organ which contains three major immunologically and biochemically distinct classes of proteins, the oa-, P-, and -y-crystallins (1).During lens growth, undifferentiated epithelial cells migrate from the anterior surface of the lens towards the equatorial axis, where they differentiate to form elongated lens fibers. Concomitant with lens cell differentiation is the activation of -y-crystallin gene expression (1, 14). The y-crystallins account for approximately 20% of the soluble lens protein. The exact function of these proteins is unknown, although the integrity of these polypeptides is generally believed to be critical to the maintenance of lens transparency (5). Changes in lens transparency that occur with age or during cataract formation are usually accompanied by structural alterations in the -y-crystallin proteins (8-10); however, the relationship between these changes and the opacification of the lens is not yet understood.Studies on rodent y-crystallins indicate that y-crystallin gene expression is differentially regulated during development (18,(21)(22)(23). Differential accumulation of low-molecular-weight human lens proteins has also been observed, suggesting that the human y-crystallin genes may also be temporally regulated during development (15,19,26). The human -y-crystallins are encoded by seven closely related genes located on chromosome 2, region q33-36 (6,7,16,17,20,24). Detailed analysis of six of these genes has revealed that two are highly related pseudogenes and contain the same in-frame termination codon at identical positions in their coding sequence (16); the other four are potentially active and encode closely related polypeptides that show 70 to 79% homology in amino acid sequence (7,17). However, the relationship between these gen...

Characterization of human lysosomal neuraminidase defines the molecular basis of the metabolic storage disorder sialidosis.

“…In vivo crystallization of proteins is a rare though naturally occuring event. It has been reported for crystallin proteins in the eye lens (Russell et al 1987) and for insulin in pancreatic acinar cells (Kuliawat and Aryan 1992). In both of these reports, the crystals are relatively small.…”

Human lens gamma-crystallins: isolation, identification, and characterization of the expressed gene products.

“…There are four potentially expressible human -y-crystallin genes (i.e., four that are not pseudogenes), so there is no a priori reason to assume that either the -yA or the -yB fraction should contain only a single polypeptide, as has been suggested in the past (45,50,54). The purity of the three monomeric crystallin fractions from size-exclusion chromatography was analyzed by isoelectric focusing and by NaDodSO4/PAGE (Fig.…”

“…Sci. USA 84 (1987) We are very grateful to Martin Breitman and John Schoenmakers for sending us copies of their manuscripts in press (32,39,40,54) prior to publication. We thank the National Diabetes Research Interchange (NDRI) for supplying the human eyes.…”

Progressive juvenile-onset punctate cataracts caused by mutation of the γD-crystallin gene

“…In particular, modifications that reduce the effective solubility of lens proteins can lead to progressive aggregation and the formation of light-scattering centers in the lens fiber cells (9,10). ␥D is one of only two ␥-crystallins expressed at high concentrations in the fiber cells of the embryonic human lens (11,12). These cells later form the lens nucleus, the site of the punctate cataract.…”