Culture medium conditioned by P19 embryonal carcinoma (EC) cells contains potent mitogenic activity which is markedly potentiated when the medium is conditioned in the presence of heparin. Fractionation of P19 medium conditioned in the presence of heparin reveals the existence of two biochemically distinct growth factor species both of which exhibit high affinity for immobilised heparin and significant activity as amphibian mesoderm-inducing agents. One of the species is recovered as a single polypeptide of apparent Mr = 15,000. This molecule is immunologically related to the protein product of the human K-FGF proto-oncogene. Transcripts derived from the murine K-FGF gene are also expressed by both differentiated and undifferentiated EC cells and embryonic stem cells. The second heparin-binding growth factor is recovered as a complex of four polypeptides, the largest of which has an apparent Mr = 17,000. This agent is immunologically and biochemically distinct from both acidic and basic fibroblast growth factor as well as K-FGF, and represents the predominant mitogenic activity in EC-cell-conditioned medium.

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