Differences in properties between human αA- and αB-crystallin proteins expressed in Escherichia coli cells in response to cold and extreme pH

It has been reported that αA-crystallin has greater protective effects against apoptosis in lens epithelial cells than αB-crystallin [Andley, Song, Wawrousek, Fleming and Bassnett (2000) J. Biol. Chem. 275, 36823–36831]. Because the αA-crystallin proteins are specifically expressed in the vertebrate lens, we examine the non-specific properties of both αA- and αB-crystallins in an Escherichia coli system. E. coli cells were transformed with the inducible protein expression vector pET-11a, harbouring the gene for either human αA- or αB-crystallin, and two other control plasmids, pET-1la vector alone or pGEX-2T vector encoding GST (glutathione S-transferase). These cells were exposed to various stress conditions, such as cold-shock at 4 °C or extremely low or high pH environments (pH 4.7 or pH 8.0) for 6 h, and survival of the host cells and the solubility of the expressed target proteins in the cytosol were examined. Under these stress conditions, the cells expressing αB-crystallin protein demonstrated significantly improved survival when compared with the other cells, and the expressed protein in the cytosol was almost soluble, in contrast with the αA-crystallin protein. Differences in the amino acid sequence between the proteins in a phenylalanine-rich region next to the N-terminal consensus α-crystallin domain was considered to be responsible for chaperone activity and cell survival.