Association properties of βB2- and βA3-crystallin:: ability to form dimers

The beta-crystallins are a major constituent of the mammalian lens, where they associate into dimers, tetramers and higher order aggregates, Appropriate association of lens crystallins is important for lens transparency, To examine the associative properties of beta B2-crystallin, we have expressed mouse beta B2-crystallin using a baculovirus system, Recombinant mouse beta B2-crystallin has an estimated monomer molecular weight of 24 kDa by SDS-PAGE, appropriate immunoreactivity and appropriate secondary structure as assessed by circular dichroism analysis, The recombinant beta B2-crystallin associates into a homodimer with a weight average molecular mass of 39 kDa, The beta B2-crystallin homodimer has an estimated K(d) of 5 x 10(-6) M, slightly greater than that of beta A3-crystallin, 0.8 x 10(-6) M. When recombinant beta B2-crystallin is combined with recombinant beta A3-crystallin, a heterodimer is formed within 10 min of incubation at room temperature, When equilibrium is reached in 4-6 h, approximately half of each crystallin associates into heterodimers. Subunit exchange between beta B2-crystallin and beta A3-crystallin occurs readily in the absence of any denaturing agents, Thus, r beta A3-r beta B2 heterodimer formation can occur under conditions similar to those found in the eye lens.