Structural basis for dimerization of the Dictyostelium gelation factor (ABP120) rod

Gelation factor (ABP120) is one of the principal actin-crosslinking protein s of Dicfyostelium discoideum. The extended molecule has an N-terminal 250- residue actin-binding domain and a rod constructed from six 100-residue rep eats that have an Ig fold. The ability to dimerize is crucial to the actin cross-linking function of gelation factor and is mediated by the rod in whi ch the two chains are arranged in an antiparallel fashion. We report the 2. 2 Angstrom resolution crystal structure of rod domains 5 and 6, which shows that dimerization is mediated primarily by rod domain 6 and is the result of a double edge-to-edge extension of beta-sheets. Thus, contrary to earlie r proposals, the chains of the dimeric gelation factor molecule overlap onl y within domain 6, and domains 1-5 do not pair with domains from the other chain. This information allows construction of a model of the gelation fact or molecule and suggests how the chains in the related molecule filamin (AB P280) may interact.