Tau-crystallin/alpha-enolase: one gene encodes both an enzyme and a lens structural protein.

Abstract

tau-Crystallin has been a major component of the cellular lenses of species throughout vertebrate evolution, from lamprey to birds. Immunofluorescence analysis of the embryonic turtle lens, using antiserum to lamprey tau-crystallin showed that the protein is expressed throughout embryogenesis and is present at high concentrations in all parts of the lens. Partial peptide sequence for the isolated turtle protein and deduced sequences for several lamprey peptides all revealed a close similarity to the glycolytic enzyme enolase (E.C. 4.2.1.11). A full-sized cDNA for putative duck tau-crystallin was obtained and sequenced, confirming the close relationship with alpha-enolase. Southern blot analysis showed that the duck genome contains a single alpha-enolase gene, while Northern blot analysis showed that the message for tau-crystallin/alpha-enolase is present in embryonic duck lens at 25 times the abundance found in liver. tau-Crystallin possesses enolase activity, but the activity is greatly reduced, probably because of age-related posttranslational modification. It thus appears that a highly conserved, important glycolytic enzyme has been used as a structural component of lens since the start of vertebrate evolution. Apparently the enzyme has not been recruited for its catalytic activity but for some distinct structural property. tau-Crystallin/alpha-enolase is an example of a multifunctional protein playing two very different roles in evolution but encoded by a single gene.

Déjà vu in proteomics. A hit parade of repeatedly identified differentially expressed proteins

“…Aside from its primary cytosolic role, enolase 1 can also be found on the cell surface serving as a plasminogen receptor [4] and in the nucleus, as an alternatively translated 37 kDa tumor suppressor c-myc promoter-binding protein (MBP-1) [5,6]. Tau-crystallin, the principal component of reptile and some avian eye lens is also coded by eno1 gene [7]. In addition, enolase 1 was reported to be a hypoxic stress protein and an HSP [8,9].…”

Identification and Gene Expression of Anaerobically Induced Enolase in Echinochloa phyllopogon and Echinochloa crus-pavonis

“…Although crystallins from embryonic duck lens possessed enolase activity, the activity was greatly reduced, corresponding with a shift from the enzymatically active dimeric form to the less active, monomeric form (Wistow and Piatigorsky, 1987). It was suggested that this shift may be due to age-related posttranslational modification of the protein (Wistow et al, 1988). In HeLa cells, yyenolase was found to be present at the centrosome throughout the cell cycle (Johnstone et al, 1992).…”

“…Severa1 instances have been described in which enolases purportedly perform nonglycolytic functions. a-Enolase has been identified as one of the crystallins, structural proteins that form the lens of the vertebrate eye (Wistow and Piatigorsky, 1987;Wistow et al, 1988). Although crystallins from embryonic duck lens possessed enolase activity, the activity was greatly reduced, corresponding with a shift from the enzymatically active dimeric form to the less active, monomeric form (Wistow and Piatigorsky, 1987).…”

Molecular cloning of cDNA and analysis of protein secondary structure of Candida albicans enolase, an abundant, immunodominant glycolytic enzyme

“…glycolytic function, enolase comprises an eye lens crystallin in avian species (60) and may function as a toxin in bacteria (2). The existence of tissue-specific forms of enolase in mammalian species and rodents suggests that enolase may have other specialized functions in these animals (26).…”