Fig. 3. Only fractions that contain the 48 kDa phosphoprotein induce MSP polymerization. (A) Coomassie-stained gels and corresponding western blots probed with antiphosphotyrosine of the vesicle and cytosol fractions of S100. The 48 kDa protein (arrowhead) is detectable only in the vesicle fraction. (B) SEM of filaments assembled on addition of the detergent-solubilized extract of membrane vesicles to cytosol in the presence of ATP. No filaments were observed when the detergent-solubilized extract was omitted. (C) Coomassie-stained gel of fractions 36-42 obtained by gel filtration chromatography of CHAPS-solubilized vesicle components. A western blot probed with antiphosphotyrosine (anti-pY) of the region containing p48 is shown below. Fractions that triggered MSP polymerization when added to cytosol + ATP are indicated by `+'; fractions that yielded no detectable filament formation are designated `-'. (D) SEM of filaments assembled by the addition of fraction 39 to cytosol + ATP. Bars in B and D, 1 µm.