THE 100 KDA F-ACTIN CAPPING PROTEIN OF DICTYOSTELIUM AMEBAS IS A VILLIN PROTOTYPE (PROTOVILLIN)

The 100 kDa actin-binding protein from Dictyostelium amoebae is an F-a ctin capping protein that displays neither severing nor crosslinking n or nucleating activities [Hofmann et al. (1992) Cell Motil. Cytoskel. 23,133-144]. Cloning and sequencing of the gene revealed that the prot ein is highly homologous to vertebrate villin, a unique component of b rush border microvilli and contains six domains fused to a villin-like headpiece domain via a threonine/proline rich neck region. The functi onal differences and similarities between the 100 kDa protein and vill in are reflected in the amino acid sequences. We draw from the data th e following conclusions. (i) The presence of a six domain protein in D ictyostelium suggests that in contrast to the current view gene duplic ations must have happened before Dictyostelium branched off during evo lution. (ii) The villin-like molecule in Dictyostelium appears to be a premature villin ('protovillin') which is able to cap actin filaments but still lacks the other villin-type actin-binding activities. This renders capping of actin filaments as the evolutionarily oldest functi on of an F-actin binding protein.