IN-VIVO MODIFICATION OF THE C-TERMINAL LYSINE OF HUMAN LENS ALPHA-B-CRYSTALIN

Both the structural and chaperone-like properties of lens alpha-crysta llins have been implicated in maintaining lens transparency. Modificat ions of lens alpha-crystallins may lead to formation of cataract by af fecting the close-packing of the crystallins or by reducing the chaper one-like activity of the cc-crystallins. A previously unreported modif ied alpha B-crystallin, whose molecular weight is 72 u greater than un modified alpha B-crystallin, has been isolated from human lenses by si ze exclusion chromatography, reversed phase HPLC and ion exchange HPLC . Approximately one nanomole of this modified alpha B-crystallin was o btained from each of five human eye lenses. Molecular weight determina tions of peptides produced by digestion with trypsin or endoproteinase Asp-N showed that the modification is in the C-terminal region of alp ha B-crystallin. The fragmentation pattern of peptides from the C-term inal region, analysed by tandem mass spectrometry, located the modific ation of the E-amino group of the C-terminal lysine. The elemental com position of this modification, determined from its exact mass, is C3H4 O2. Because this modification decreases the net charge of alpha B-crys tallin by one unit, and because the C-terminus has been implicated in the chaperone activity attributed to alpha B-crystallin, this modifica tion at Lys 175 may have a significant role in cataractogenesis. (C) 1 997 Academic Press Limited.