Evolutionary and functional relationships between the basic and acidic β-crystallins

Abstract

beta-Crystallins are complex oligomers composed of many related subunits. In order to understand their interactions we have built molecular models of several bovine beta-crystallins, based on their sequence similarity to the well-defined gamma-II crystallin structure, using interactive computer graphics techniques. Their common origin with gamma-crystallin is displayed in both the retention of four-fold sequence repeats of critical residues involved with stabilizing a folded beta-hairpin and the conservation of core-filling hydrophobic side-chains. The beta-crystallins have been built as bilobal molecules with each domain composed of two 'Greek key' motifs which associate about an approximate two-fold axis to form beta-sheets. The beta-crystallin sequences have previously been shown to comprise two families, the basic and acidic subunits, which have extensions of sequence. The three-dimensional models show how the two families appear to stabilize the folded beta-hairpin in the N- and C-terminal domains in ways which suggest that they have diverged from a common ancestor in different ways. Acidic beta-crystallins, like gamma-crystallins, have a regular array of charges on their N-terminal domain which has been interrupted in basic beta-crystallins by hydrophobic residues which may be related to the presence of a C-terminal extension. beta-Crystallins are more highly charged than gamma-crystallins although their charge density is higher in certain regions of the N-terminal domain, particularly in beta B1-crystallin. beta-crystallins also differ from gamma-crystallins in the virtual absence of core-filling sulphydryl groups whereas they have numerous sulphur-containing side-chains together with tryptophan and histidine rings protruding from the globular domains, particularly in the acidic subunits. The burial of these residues in subunit contacts is consistent with their spectroscopic and electrostatic properties. Protein subunit aggregation commonly occurs through hydrophobic interaction or beta-sheet extension. Analysis of the subunit surfaces has identified an N-terminal hydrophobic region common to beta B1 and beta B2 whereas a C-terminal hydrophobic loop region is common to beta B1 and beta A1 and may be correlated with their association properties. It is suggested that the polar C-terminal domain of beta B2 contributes towards the solubility of higher aggregates by interactions involving beta-sheet structure.