Having worked in a lens protein biochemistry and biophysics lab during my Ph.D years, a corner of my heart remained (and remains) fond of the lens crystallins. One such crystallin, alpha crystallin, had already been demonstrated to be a molecular chaperone by the time we began this piece of work, when I was a PI with a lab of my own. This protein shows homology to many small heat shock proteins, but its structure has never been determined. In the chain of one of the alpha crystallins, known as alpha-B, the first sixty-odd amino acids are followed by a nearly-120 residues-long small heat shock domain. We decided to make just the small heat shock domain from purified alpha-B and examine its structural and conformational behaviour. We did not yet have enough funds to make recombinant proteins and so we purified alpha crystallin from bovine lenses and hit upon a neat method of separating the first sixty-odd amino acids from the heat shock domain using cyanogen-bromide cleavage. This method essentially breaks peptide bonds that follow the residue methionine. Fortuitously, in the sequence of alpha-B crystallin, the only methionine residue is located exactly at the domain boundary separating the first stretch of sixty-odd residues from the heat shock domain. So, we cleaved the domain and purified it and studied it. Unlike alpha crystallin which forms hollow 40-mer spheres, the excised heat shock domain formed only trimers. While there was high surface hydrophobicity, and also well-formed beta sheet structure within these trimers, there was no chaperone activity against thermal aggregation of other proteins of the type seen with alpha crystallin. Rather, the domain itself displayed aggregation upon heating. This, and related insights, are presented in this paper.

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