To investigate the role of phosphorylation of a precursor form of interleukin-1α (pre-IL-1α), we obtained cells producing either phosphorylated or unphosphorylated pre-IL-1α. Although calcium-dependent proteolytic processing of unphosphorylated pre-IL-1α could be observed in cell lysates, proteolytic processing was not induced by treatment with calcium ionophore in intact cells producing the unphosphorylated pre-IL-1α. Further, unphosphorylated pre-IL-1α showed no calcium-dependent binding to acidic phospholipids at concentrations below 5 × 10−6 M. These results suggested that phosphorylated pre-IL-1α became susceptible to proteolytic processing by association with the cell membrane in a calcium-dependent manner.
