Autocrine/paracrine activator of cell proliferation: purification of a 4-6 kD compound with growth-factor-like effects in Tetrahymena thermophila.

Extracellular medium from cultures of Tetrahymena thermophila was collected, concentrated and fractionated by ultrafiltration and low-pressure column chromatography. The effect on proliferation of various fractions was tested on cultures of T. thermophila in synthetic medium at low initial cell densities (< 250 cells/ml). In unsupplemented cultures, cells failed to thrive and died before proliferation. Addition of the fraction isolated from the extracellular medium in the molecular weight range of 1-10 kD activated the cells to proliferate. The size of the main compound in this fraction, herein referred to as TPAF, was in the range of 4-6 kD as estimated by SDS-PAGE. Evidence of a dialysable factor produced by cells in the stationary phase inhibiting cell proliferation has also been found.