Hierarchy of lens proteins requiring protection against heat-induced precipitation by the α crystallin chaperone

Gel filtration of the water-soluble extract from bovine lens yields a group of proteins, emerging between the peaks of β(H) and β(L) crystallins, which show a considerably greater sensitivity to heat-induced aggregation/precipitation than the far more abundant β and γ crystallins. However, the small heat shock protein: α crystallin was effective in protecting these trace constituents of the lens from precipitating out of solution at 55 °C (measured under the standard conditions in a pH 7.5 buffer containing 50 mM sodium phosphate, 100 mM NaCl, 1 mM EDTA and 0.05% NaN₃). Prominent components of the precipitate, formed in the absence of a recombinant αB crystallin chaperone could be resolved by one- and two- dimensional electrophoresis. Identification by amino acid sequencing revealed that the heat-sensitive group of lens proteins comprised glyceraldehyde-3- phosphate dehydrogenase (M(r) ~ 39 kDa), enolase (~ 48 kDa), leucine aminopeptidase (~ 52 kDa) and aldehyde dehydrogenase (~ 53 kDa). These findings indicate for the first time that the aggregation of such minor lens constituents could possibly contribute to initiating the process of opacification in the development of cataracts.