SECRETION FROM PERMEABILIZED MAST-CELLS IS ENHANCED BY ADDITION OF GELSOLIN - CONTRASTING EFFECTS OF ENDOGENOUS GELSOLIN

Permeabilised rat mast cells were exposed to gelsolin and its N-termin al half (S1-3), proteins that sever actin filaments in a calcium-depen dent acid independent manner, respectively, Gelsolin and S1-3 induced a decrease in cellular F-actin content acid an increase in the extent of the secretory response, The calcium sensitivities of both these eff ects were consistent with the differential calcium requirements of the two proteins. Segment 1 (S1), which binds G-actin and caps filaments but does not sever them, did not show these effects, Thus, secretion o f mast cells is promoted as a consequence of the severing activity of exogenous gelsolin or S1-3. Most of the endogenous gelsolin remained w ithin permeabilised, washed mast cells and its distribution in resting state was predominantly cortical, Addition of calcium in the absence of MgATP did not reduce the F-actin content; by contrast, calcium with MgATP induced E-actin loss that was unaffected by the presence of ant i-gelsolin. Because this antibody inhibits the severing activity of ge lsolin, these results indicate that in permeabilised mast cells the se vering activity of the remaining endogenous gelsolin is not involved i n cortical actin filaments disassembly. Upon exposure to GTP-gamma-S i n the absence of calcium, the content of cortical gelsolin was reduced , This parallels our previous observation of a GTP-gamma-S induced red uction of cortical actin filaments followed by their relocation to the cell's interior (Norman et al, (1994) J. Cell Biol, 126, 1005-1015) a nd suggests that actin redistribution may be a consequence of dissocia tion of gelsolin caps brought about by activation of a GTP-binding pro tein.