THE ACTIN-BINDING PROTEIN COMITIN (P24) IS A COMPONENT OF THE GOLGI-APPARATUS

Comitin (p24) was first identified in Dictyostelium discoideum as a me mbrane-associated protein which binds in gel overlay assays to G and F actin. To analyze its actin-binding properties we used purified, bact erially expressed comitin and found that it binds to F actin in spin d own experiments and increases the viscosity of F actin solutions even under high-salt conditions. Immunofluorescence studies, cell fractiona tion experiments and EM studies of vesicles precipitated with comitin- specific monoclonal antibodies showed that comitin was present in D. d iscoideum on: (a) a perinuclear structure with tubular or fibrillary e xtensions; and (b) on vesicles distributed throughout the cell. In imm unofluorescence experiments using comitin antibodies NIH 3T3 fibroblas ts showed a similar staining pattern as D. discoideum cells. Using bon a fide Golgi markers the perinuclear structure was identified as the G olgi apparatus. The results were supported by an electron microscopic study using cryosections. Based on these data we propose that also in Dictyostelium the stained perinuclear structure is the Golgi apparatus . In vivo the perinuclear structure was found to be attached to the ac tin and the microtubule network. Alteration of the actin network or de polymerization of the microtubules led to its dispersal into vesicles distributed throughout the cell. These results suggest that the Golgi apparatus in D. discoideum is connected to the actin network by comiti n. This protein seems also to be present in mammalian cells.