We have expressed in Escherichia coli a construct corresponding to sequence repeats 5 and 6 of the rod domain of the actin-binding protein Dictyostelium gelation factor (ABP-120), We have obtained orthorhombic P2(1)2(1)2(1) crystals of the protein with a = 43.5 Angstrom, b = 103.2 Angstrom, c = 124.4 Angstrom. These crystals diffract past 2.2 Angstrom resolution using synchrotron radiation and are suitable for high-resolution structural analysis. ABP-120 is a key component of the Dictyostelium cytoskeleton, where it functions to crosslink. F-actin filaments into networks, This crosslinking function of ABP-120 depends crucially on the formation of dimeric molecules that contain an actin-binding site on each chain, and this dimerization is brought about through interactions between repeating sequence modules in the rod domain, Because the construct we have expressed retains the ability to dimerize, it should enable us to establish the precise manner in which these sequence repeats interact with one another in the intact molecule. (C) 1997 Academic Press.