Phalloidin and tropomyosin do not prevent actin filament shortening by the 90 kD protein-actin complex from brain.

Previously we reported the purification from bovine brain of the 90 kD protein-actin complex that shortens actin filaments. In the present work we study the effect of this complex on actin polymerized in the presence of phalloidin (PL) or tropomyosin (TM) which are known to stabilize actin filaments. The effect of the complex has been compared with that of cytochalasin D (CD), a fungal metabolite that also shortens actin filaments. Low shear viscosimetry and electron microscopy showed that PL or TM could not prevent the shortening of actin filaments in the presence of 90 kD protein-actin complex whereas they effectively protected actin filaments from shortening by CD. We conclude that the 90 kD protein-actin complex is a more potent filament-shortening factor than CD.