Multiple molecular forms of murine thymocyte-stimulating factor

Murine thymocyte stimulating factor (TSF) was found to sediment in sucrose density gradients on a broad band with peaks at about 2.60 S and 2.0 S. Two main peaks of TSF activity (with buoyant densities of 1.34 and 1.28 g/ml) were found in CsCl density gradients. Gel chromatography on Sephadex G-100 columns of the material sedimented in sucrose or CsCl density gradients originated multiple peaks of TSF activity with various molecular weights. Heterogeneity of molecular forms of TSF was also found upon dilution of the factor. The lowest molecular weights found were 4000 and 4700 daltons. When Sephadex fractions containing the low molecular weight material were pooled and rerun on Sephadex columns, molecular species with a wide range of molecular weights were found. Temperature also affects the appearance of the low molecular weight forms of TSF. Most of the experiments presented in this work were carried out with Sephadex-purified TSF. Multiple molecular forms, however, and, in particular, the forms with molecular weights of 4000 and 4700 daltons were found also with TSF-Fraction IIIa, a highly purified preparation of this factor.