Abstract
From the pharyngeal baskets of the ascidians Microcosmus sulcatus and Phallusia mammilata we have purified an 85-kDa protein that is characterized as a member of the gelsolin family. These proteins from both species show the same behaviour in functional assays. The ascidian gelsolin binds two actin monomers in a highly cooperative manner. This complex formation is Ca 2+-dependent, but not completely reversible, as on removal of Ca 2+ one actin monomer dissociates leaving a 1:1 complex between gelsolin and G-actin. The properties of F-actin severing and G-actin nucleation depend on the presence of free Ca 2+ in a micromolar range, with half maximum activation at about 3×10 −6 M. The protein becomes inactivated when Ca 2+ concentrations of 0.5 mM are exceeded. Fragmentation of F-actin by the ascidian gelsolin is comparably fast to that of vertebrate gelsolin. A steady state of actin fragmentation is reached within 2–4 s. Promotion of G-actin nucleation is also comparable to that of vertebrate gelsolin. Regarding functional aspects, the ascidian gelsolin is more closely related to vertebrate gelsolin than to an arthropod gelsolin from crayfish tail muscle.
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