H-1-NMR SPECTROSCOPY OF BETA-B2-CRYSTALLIN FROM BOVINE EYE LENS - CONFORMATION OF THE N-TERMINAL AND C-TERMINAL EXTENSIONS

H-1-NMR spectroscopic studies of a 46-kDa homodimer, betaB2-crystallin, from bovine eye lens are presented. BetaB2-crystallin has terminal extensions extending from globular N- and C-terminal domains that are well resolved in the NMR spectra, whereas, in the main, resonances from the bulk of the protein are not observed. Using two-dimensional NMR methods on betaB2-crystallin, its synthesised terminal extensions and a proteolysed sample of betaB2-crystallin with a portion of its C-terminus removed, it was possible to assign resonances to most of the amino acids in the terminal extensions. One-dimensional experiments at various pH values provided H-2 chemical shifts for the three terminal extension histidines from which their pK(a) values were measured. It is concluded that the terminal extensions appear to be of little ordered conformation, are accessible to solvent and flex freely from the main body of the protein. The results of the NMR spectroscopic studies of betaB2-crystallin are in excellent agreement with those for the X-ray crystal structure [Bax, B., Lapatto, R., Nalini, V., Driessen, H., Lindley, P. F., Mahadevan, D., Blundell, T. L. & Slingsby, C. (1990) Nature 347, 776-780]. No change in the spectrum of betaB2-crystallin was observed in the presence of calcium, suggesting that the termini are not involved in calcium binding.