Association Behaviour of Human βB1-Crystallin and its Truncated Forms

βB1-crystallin plays an important role in the assembly of βH-crystallin yet is known to be subject to N-terminal sequence truncations during human lens development and ageing. Here we have over-expressed human βB1-crystallin, and various truncated forms in Escherichia coli and used mass spectrometry to monitor the monomer molecular weight. Gel permeation chromatography and laser light scattering have been used to estimate the assembly size of the various polypeptides as a function of protein concentration. The full-length βB1-crystallin behaves as a dimer, like recombinant human βB2-crystallin, but undergoes further self-association at high protein concentrations, unlike the βB2-crystallin. Major truncations from the N-terminal extension lead to anomalous behaviour on gel permeation chromatography indicative of altered interactions with the column matrix, whereas light scattering indicated dimers at low protein concentration that self-associate as a function of protein concentration. Loss of 41 residues from the N-terminus, equivalent to an in vivo truncation site, resulted in temperature-dependent phase separation behaviour of the shortened βB1-crystallin. Good crystals have been grown of a truncated version of human βB1-crystallin using an in vitro cleavage protocol.