We characterized the properties of Drosophila melanogaster DAAM-FH2 and DAAM-FH1-FH2
fragments and their interactions with actin and profilin by using various biophysical
methods and in vivo experiments. The results show that although the DAAM-FH2 fragment
does not have any conspicuous effect on actin assembly in vivo, in cells expressing
the DAAM-FH1-FH2 fragment, a profilin-dependent increase in the formation of actin
structures is observed. The trachea-specific expression of DAAM-FH1-FH2 also induces
phenotypic effects, leading to the collapse of the tracheal tube and lethality in
the larval stages. In vitro, both DAAM fragments catalyze actin nucleation but severely
decrease both the elongation and depolymerization rate of the filaments. Profilin
acts as a molecular switch in DAAM function. DAAM-FH1-FH2, remaining bound to barbed
ends, drives processive assembly of profilin-actin, whereas DAAM-FH2 forms an abortive
complex with barbed ends that does not support profilin-actin assembly. Both DAAM
fragments also bind to the sides of the actin filaments and induce actin bundling.
These observations show that the D. melanogaster DAAM formin represents an extreme
class of barbed end regulators gated by profilin.
