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Vol. 13, Issue 11, 3811-3821, November 2002
and
Programs in *Cellular Biotechnology and Twinfilin is a ubiquitous and abundant actin monomer-binding
protein that is composed of two ADF-H domains. To elucidate the role of
twinfilin in actin dynamics, we examined the interactions of mouse
twinfilin and its isolated ADF-H domains with G-actin. Wild-type
twinfilin binds ADP-G-actin with higher affinity
(KD = 0.05 µM) than ATP-G-actin
(KD = 0.47 µM) under physiological ionic conditions and forms a relatively stable
(koff = 1.8 sStructural
Biology, Institute of Biotechnology, University of Helsinki, 00014, Helsinki, Finland; and MRC Laboratory of Molecular
Biology, Cambridge, CB2 2QH, England
1) complex
with ADP-G-actin. Data from native PAGE and size exclusion chromatography coupled with light scattering suggest that twinfilin competes with ADF/cofilin for the high-affinity binding site on actin
monomers, although at higher concentrations, twinfilin, cofilin, and
actin may also form a ternary complex. By systematic deletion analysis,
we show that the actin-binding activity is located entirely in the two
ADF-H domains of twinfilin. Individually, these domains compete for the
same binding site on actin, but the C-terminal ADF-H domain, which has
>10-fold higher affinity for ADP-G-actin, is almost entirely
responsible for the ability of twinfilin to increase the amount of
monomeric actin in cosedimentation assays. Isolated ADF-H domains
associate with ADP-G-actin with rapid second-order kinetics, whereas
the association of wild-type twinfilin with G-actin exhibits kinetics
consistent with a two-step binding process. These data suggest that the
association with an actin monomer induces a first-order conformational
change within the twinfilin molecule. On the basis of these results, we
propose a kinetic model for the role of twinfilin in actin dynamics and its possible function in cells.
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