Mass measurements of C-terminally truncated alpha-crystallins from two-dimensional gels identify Lp82 as a major endopeptidase in rat lens.

Molecular chaperone activity of lens alpha-crystallins is reduced by loss of the C terminus. The purpose of this experiment was to 1) determine the cleavage sites produced in vitro by ubiquitous m-calpain and lens-specific Lp82 on alpha-crystallins, 2) identify alpha-crystallin cleavage sites produced in vivo during maturation and cataract formation in rat lens, and 3) estimate the relative activities of Lp82 and m-calpain by appearance of protease-specific cleavage products in vivo. Total soluble protein from young rat lens was incubated with recombinant m-calpain or Lp82 and 2 mM Ca2+. Resulting fragmented alpha-crystallins were separated by two-dimensional gel electrophoresis. Eluted alpha-crystallin spots were analyzed by mass spectrometry. Cleavage sites on insoluble alpha-crystallins were determined similarly in mature rat lens nucleus and in cataractous rat lens nucleus induced by selenite. In vitro proteolysis of alphaA-crystallin by Lp82 and m-calpain produced unique cleavage sites by removing 5 and 11 residues, respectively, from the C terminus. In vivo, the protease-specific truncations removing 5 and 11 residues from alphaA were both found in maturing lens, whereas only the truncation removing 5 residues was found in cataractous lens. Other truncation sites, common to both calpain isoforms, resulted from the removal of 8, 10, 16, 17, and 22 residues from the C terminus of alphaA. Using uniquely truncated alphaA-crystallins as in vivo markers, Lp82 and m-calpain were both found to be active during normal maturation of rat lens, whereas Lp82 seemed especially active during selenite cataract formation. These C-terminal truncations decrease chaperone activity of alpha-crystallins, possibly leading to the observed increases in insoluble proteins during aging and cataract. The methodology that allowed accurate mass measurements of proteins eluted from 2D gels should be useful to examine rapidly other post-translational modifications.

Lens proteomics: analysis of rat crystallin sequences and two-dimensional electrophoresis map.

PURPOSE: To determine the sequence of four rat beta-crystallins, confirm the sequences by mass spectrometry, and produce a two-dimensional electrophoresis (2-DE) map of soluble crystallins in young rat lens. METHODS: New or additional sequences were determined for betaB1, betaB3, betaA3, and betaA4-crystallin cDNAs from Sprague-Dawley rats, and the deduced protein sequences confirmed by mass spectrometry. The identity and relative abundance of each crystallin was then determined by 2-DE of soluble protein from whole lenses of 12-day-old rats, image analysis, and tandem mass spectrometry (MS/MS) spectra of peptides from in-gel digests. RESULTS: The previously unreported sequence of rat betaA4 cDNA encoded a 195-amino-acid protein. Additional cDNA sequencing provided the previously unknown N-terminal sequence of rat betaA3, found two differences from the previous amino acid sequences of both rat betaB1 and betaB3, and detected a polymorphism at residue 54 in rat betaB3. These new sequences were then confirmed by whole protein masses and MS/MS spectra of proteolytic digests. 2-DE analysis provided a more detailed map of rat crystallins than previously available and allowed the composition of crystallins in young rat lens to be compared with that in young human lens. CONCLUSIONS: This report provides baseline data that will facilitate the analysis of posttranslational modifications in rat crystallins during cataract. Detection of a polymorphism in the sequence of rat betaB3 suggests that crystallins in humans could also exhibit polymorphisms. The unusual abundance of rat betaB3 and low abundance of betaB2 may account for the increased susceptibility of rat crystallins to insolubilization during aging and cataract.