ASSOCIATION PROPERTIES OF BETA-B2-CRYSTALLIN AND BETA-A3-CRYSTALLIN -ABILITY TO FORM DIMERS

The beta-crystallins are a major constituent of the mammalian lens, wh ere they associate into dimers, tetramers and higher order aggregates, Appropriate association of lens crystallins is important for lens tra nsparency, To examine the associative properties of beta B2-crystallin , we have expressed mouse beta B2-crystallin using a baculovirus syste m, Recombinant mouse beta B2-crystallin has an estimated monomer molec ular weight of 24 kDa by SDS-PAGE, appropriate immunoreactivity and ap propriate secondary structure as assessed by circular dichroism analys is, The recombinant beta B2-crystallin associates into a homodimer wit h a weight average molecular mass of 39 kDa, The beta B2-crystallin ho modimer has an estimated K-d of 5 x 10(-6) M, slightly greater than th at of beta A3-crystallin, 0.8 x 10(-6) M. When recombinant beta B2-cry stallin is combined with recombinant beta A3-crystallin, a heterodimer is formed within 10 min of incubation at room temperature, When equil ibrium is reached in 4-6 h, approximately half of each crystallin asso ciates 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 und er conditions similar to those found in the eye lens.