Human macrophage colony stimulating factor (M-CSF): alternate RNA splicing generates three different proteins that are expressed on the cell surface and secreted.
Human macrophage colony stimulating factor (M-CSF) cDNA clones were isolated from a pancreatic carcinoma cell line. Three different classes of M-CSF precursor protein (256, 554 and 438 amino acids in length) were predicted to be encoded by these cDNAs. Two of these, that we designate M-CSF alpha and M-CSF beta have already been described. The third, M-CSF gamma represents a novel class of M-CSF cDNA. All three precursors share a 32 amino acid signal sequence and the first 149 amino acids of the mature protein. At this position, M-CSF beta and gamma have insertions of 298 and 182 amino acids relative to M-CSF alpha. The first 182 amino acids of these insertions are shared between M-CSF beta and gamma. All three precursors share the C-terminal 75 amino acids that encode the transmembrane and cytoplasmic domains. Expression of all three cDNAs in COS-7 monkey kidney cells gave rise to soluble M-CSF activity, associated with proteins of subunit molecular weight 44 Kda (beta and gamma) or 28 Kda (alpha). In addition, M-CSF proteins could be detected on the surface of the transfected cells by indirect immunofluorescence. When the transmembrane and cytoplasmic domains of M-CSF alpha were removed by introducing a stop codon after amino acid 190, no membrane-bound M-CSF could be detected, but the truncated protein was secreted efficiently and was biologically active. This suggests that all three forms of M-CSF can exist as cell surface proteins, anchored by their hydrophobic transmembrane domains, and can be processed to soluble forms by proteolytic digestion. Although all soluble forms of M-CSF were biologically active in murine bone marrow colony and proliferation assays, they showed greatly reduced or no activity in similar assays using human bone marrow.
Cosman
D
D
Department of Molecular Biology, Immunex Corporation, Seattle, Washington 98101.
Wignall
J
J
Anderson
D
D
Tushinski
J
J
Gallis
B
B
Urdal
D
D
Cerretti
D P
DP
D000595:Amino Acid Sequence; D000818:Animals; D001483:Base Sequence; D002460:Cell Line; D003115:Colony-Stimulating Factors / Q000235:genetics*; D004273:DNA, Neoplasm / Q000235:genetics*; D005091:Exons; D005796:Genes; D005813:Genes, Synthetic; D006801:Humans; D016173:Macrophage Colony-Stimulating Factor; D008969:Molecular Sequence Data; D010190:Pancreatic Neoplasms; D012326:RNA Splicing; D014162:Transfection
D003115:Colony-Stimulating Factors; D004273:DNA, Neoplasm; D016173:Macrophage Colony-Stimulating Factor