Characterization of receptors for a T cell derived cytosolic BCGF-60kD

A 60 kD cytosolic protein derived from human peripheral blood T cells has been shown to mediate proliferation in BCGF responsive long term B cell cultures. In this report, we show cBCGF-60kD can be radioiodinated with no significant loss of biologic activity and bound specifically to B cell surface membrane receptors. The time course of ¹²⁵I-cBCGF-60kD demonstrated that binding reached saturation within 40 minutes, (T 1/2 of 15 minutes). Scatchard analyses of ligand-receptor binding identified a high affinity receptor (0.6-3 x 10³ receptors per cell) having a dissociation constant of 1-8 pM. Partial cross-competition experiments with a secreted, T cell derived BCGF-12kD identified a potential receptor heterogeneity on BCGF responsive B cell line.