Comparative studies of beta s-crystallins from human, bovine, rat and rabbit lenses

Soluble extracts from young bovine, human, rat and rabbit lenses were fractionated by high resolution size-exclusion chromatography to demonstrate the existence of three discrete size-classes of monomeric crystallins in each species. These were identified by ion exchange chromatography, amino acid analysis, SDS electrophoresis and isoelectric focusing as the beta s-, gamma A- and gamma B-crystallins. Conventional SDS electrophoretic analysis of these proteins revealed apparent Mr values of about 23kD, 22kD and 19kD, respectively. Similar analysis in the presence of 6 M urea showed the proteins all co-migrated with an apparent Mr of about 20,500, which is far more consistent with the molecular weights calculated from beta s- and gamma-crystallin sequence data. Amino acid compositions of all the beta s samples indicate a high degree of homology to the bovine protein, whose sequence is known. The different species beta s-crystallins showed other general similarities in size, charge, thiol content and secondary structural properties. On the other hand, near UV CD and fluorescence emission and energy transfer measurements indicate that these proteins have subtle yet significant differences in their tertiary structures. Unlike the gamma-crystallins, the secondary structure of all of the beta s samples is completely denatured in the presence of 8 M urea at 20 degrees C.