A gradient in the density of intramembrane particles is formed during capping induced by concanavalin A

During capping of concanavalin A (ConA) by amoebae of Dictyostelium discoideum, each cell becomes polarized, with the ConA at one end and newly extended pseudopodia at the opposite end of the cell. This new polarity is stable until the cap is shed or internalized. Intramembrane particles (IMPs) are widely believed to represent large integral membrane proteins, many of which are ion pumps and channels. Since asymmetric ion currents have been implicated in the development of cell polarity, we have used morphological landmarks associated with the capped cells in freeze-fracture to make a morphometric analysis of the IMP distribution relative to the axis of polarization of the capped cell. Untreated cells in suspension extend pseudopodia randomly from their surfaces. In these cells the numerical density of IMPs is random. However, capped cells demonstrate a density gradient of IMPs with the lowest density usually in the pseudopodia and the highest in the cap. The difference in density between the cap and other regions of the cell is two- to threefold for all IMPs, but can be as much as sevenfold for greater than 12 nm IMPs. This study is the first to document that the numerical density of IMPs is altered in response to ligand-induced capping and demonstrates that the distribution of IMPs in a capped cell is related to the axis of polarization of the cell. These results suggest that the development of cell polarity during capping in Dictyostelium amoebae may be due to the asymmetric distribution of IMPs, which may cause asymmetric ion currents across the cell.