Evidence for an intracellular precursor for human B-cell growth factor

Human B-cell growth factor has been described as a trypsin-sensitive protein of M(r) 12,000-14,000. Evidence is provided herein that this relatively low molecular weight product may be released from a larger precursor molecule of M(r) 60,000-80,000. The precursor protein is confined to the freshly isolated T lymphocytes, and only the M(r) 12,000-14,000 moiety is released upon lectin stimulation. The precursor protein was subjected to limited tryptic digestion, which demonstrated that the biologically active fraction of the moiety resided in a relatively low molecular weight fragment. The T lymphocyte routinely possessed an intracytoplasmic pool of the precursor protein, the amount of which cyclically varied depending upon its depletion by the secretion process of a lower molecular weight product. Analysis of the mRNA size coding for the majority of B-cell growth factor activity, determined by translation in Xenopus laevis oocytes, suggested that the B-cell growth factor-specific mRNA resided in the ≥15S range. This value is consistent with the size of the larger precursor. Therefore, it is proposed that a precursor-product relationship exists for the processing of human B-cell growth factor, analogous to that which has been described for several other cytokines.