Presence of a novel inhibitor of capping protein in neutrophil extract

Capping of actin filament barbed ends regulates the duration of filament elongation and the steady-state level of actin polymerization. We find that the specific capping activity (capping activity per milligram protein) increased when a high speed supernatant of lysed neutrophils was diluted with buffer. The specific capping activity also increased when the concentration of barbed ends increased. This suggested the presence of a capping protein inhibitor that dissociates from capping protein upon dilution and that competes with barbed ends for binding to capping protein. Gel filtration of supernatant revealed a fraction of low-molecular-weight inhibitor (separated from capping protein) that both inhibited and reversed capping of barbed ends by pure capping protein. The properties and molecular weight of this inhibitor do not match with those of other inhibitors including V-1, VASP, or CARMIL. Thus, this inhibitor must either be a modified version of a known inhibitor or a novel inhibitor of capping.