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We previously reported that the jumyo mutant of a cilate protozoan Paramecium tetraurelia excretes into the medium a factor which promotes its own cell division. Here, the factor was purified to electrophoretic homogeneity through a series of liquid chromatographic procedures. This substance is a protein with a molecular weight of 17,000 which at concentrations of 1 X 10(-9) M (17 ng/ml) or more results in the recovery of the cell division rate of the jumyo mutant to the level of the wild type. The factor is therefore considered to be a growth factor and was named Paramecium growth factor (ParGF). This is the first report of direct proof for the production of a growth factor in an organism other than vertebrates.

The temperature-specific G surface antigen of Paramecium primaurelia strain 156 was biosynthetically labeled by [3H]myristic acid in its membrane-bound form, but not in its soluble form. It could be cleaved by a phosphatidylinositol-specific phospholipase C from Trypanosoma brucei or from Bacillus cereus which released its soluble form with the unmasking of a particular glycosidic immunodeterminant called the crossreacting determinant. The Paramecium enzyme, capable of converting its membrane-bound form into the soluble one, was inhibited by a sulphydril reagent in the same way as the trypanosomal lipase. From this evidence we propose that the Paramecium temperature-specific surface antigens are anchored in the plasma membrane via a glycophospholipid, and that an endogenous phospholipase C may be involved in the antigenic variation process.