The adhesion modulation protein, AmpA localizes to an endocytic compartment andinfluences substrate adhesion, actin polymerization and endocytosis invegetative Dictyostelium cells

Abstract

Abstract Background AmpA is a secreted 24Kd protein that has pleiotropic effects onDictyostelium development. Null mutants delay development atthe mound stage with cells adhering too tightly to the substrate. Prestalkcells initially specify as prespore cells and are delayed in their migrationto the mound apex. Extracellular AmpA can rescue these defects, but AmpA isalso necessary in a cell autonomous manner for a nteriorl ike c ells (ALCs) to migrate to the upper cup. The ALCsare only 10% of the developing cell population making it difficult to studythe cell autonomous effect of AmpA on the migration of these cells. AmpA isalso expressed in growing cells, but, while it contains a hydrophobic leadersequence that is cleaved, it is not secreted from growing cells. This makesgrowing cells an attractive system for studying the cell autonomous functionof AmpA. Results In growing cells AmpA plays an environment dependent role in cell migration.Excess AmpA facilitates migration on soft, adhesive surfaces but hindersmigration on less adhesive surfaces. AmpA also effects the level of actinpolymerization. Knockout cells polymerize less actin while over expressingcells polymerize more actin than wild type. Overexpression of AmpA alsocauses an increase in endocytosis that is traced to repeated formation ofmultiple endocytic cups at the same site on the membrane. Immunofluorescenceanalysis shows that AmpA is found in the Golgi and colocalizes with calnexinand the slow endosomal recycling compartment marker, p25, in a perinuclearcompartment. AmpA is found on the cell periphery and is endocyticallyrecycled to the perinuclear compartment. Conclusion AmpA is processed through the secretory pathway and traffics to the cellperiphery where it is endocytosed and localizes to what has been defined asa slow endosomal recycling compartment. AmpA plays a role in actinpolymerization and cell substrate adhesion. Additionally AmpA influencescell migration in an environment dependent manner. Wild type cells show verylittle variation in migration rates under the different conditions examinedhere, but either loss or over expression of AmpA cause significant substrateand environment dependent changes in migration.