Fig. 4. p48 immunoprecipitated with antiphosphotyrosine induces MSP polymerization when recombined with cytosol. (A) Coomassie-stained gel of S100 (lane 1) and the material obtained by immunoprecipitation with antiphosphotyrosine (lane 2). Lane 3 shows a western blot of the immunoprecipitate probed with antiphosphotyrosine. The two dark bands have M_rs corresponding to IgG heavy (HC) and light chains (LC) and are stained by the secondary antibody used to develop the western blot. To confirm that the band just below HC is p48, S100 was incubated with ³²P--ATP, immunoprecipitated with antiphosphotyrosine and analyzed by autoradiography (lane 4). An immunoprecipitated band at M_r 48 kDa is heavily labeled. The fainter band at M_r 45 kDa was not seen on other antiphosphotyrosine western blots (see Fig. 1C and Fig. 2A,C) and is probably a breakdown product of p48. (B) SEM of filaments obtained when immunoprecipitated p48 was combined with cytosol and ATP. Bar, 1 µm.