THE ACTIN MONOMERS IN THE TERNARY GELSOLIN - 2 ACTIN COMPLEX ARE IN AN ANTIPARALLEL ORIENTATION

Gelsolin forms ternary complexes with two actin monomers in the presen ce of Ca2+, which nucleate actin polymerization and cap the barbed end s of filaments. It has therefore been assumed that the two actins are oriented in a similar manner to the terminal subunits in the genetic h elix of F-actin. We have tested this using chemical cross-linking with N,N'-1,4-phenylenedimaleimide. For all conditions tested, we identifi ed as the only cross-linked dimeric species an actin dimer indistingui shable from the lower actin dimer of 86 kDa. This lower dimer was prev iously identified in the initial phase of actin polymerization and als o when actin paracrystals are chemically cross-linked [Millonig, R., S alvo, H. & Aebi, U. (1988) J. Cell Biol. 106, 785-796]. It probably de fines a contact between adjacent monomers oriented in an antiparallel orientation. In contrast, when F-actin is cross-linked by the same rea gent, an upper dimer of apparent molecular mass 115 kDa is formed, whi ch corresponds to adjacent monomers in the genetic helix. The formatio n of this upper dimer was specifically inhibited by addition of gelsol in to F-actin. Evidence is presented for a Cys374-Cys374 cross-link in the lower dimer. Isolated lower dimer binds to gelsolin in a 1:1 stoi chiometry, but it inhibits nucleation of polymerization by gelsolin. O ther gelsolin constructs that bind two actin subunits (e.g. the N-term inal half of the molecule, which has severing and capping but no nucle ating activity) also form only lower dimer when cross-linked with N,N' -1,4-phenylenedimaleimide. Only segment 2-6 (gelsolin fragment devoid of the N-terminal segment 1) induces an upper dimer orientation of the two actins under nucleating conditions. Our evidence suggests that th e two actins associated with gelsolin are not fixed in the orientation of adjacent subunits in F-actin; instead they have a flexible orienta tion with respect to each other, which permits cross-linking into a st able antiparallel form that does not correspond to the presumed nuclea ting conformation.