Reassessment of the murine c-fms proto-oncogene sequence.

The c-fms gene encodes the receptor for the macrophage-colony stimulating factor (M-CSF) which promotes the proliferation and survival of cells of the monocytic lineage (1). A cDNA library was constructed from a murine myeloblastic leukemic cell line, BMC-4, in which the c-fms gene was overexpressed (2). Eleven independent full-length cDNAs were isolated after screening with a probe encompassing the c-fms first coding exon. The coding region of two arbitrary chosen clones was sequenced on both strands by the Sanger method. The sequence of the two clones was identical but different from the available murine c-fms sequence (3), by 24 differences at the nucleotide level which led to 12 amino acid changes. Comparison with the rat, feline and human sequences (4, 5, 6) showed that 11 of these 12 changed amino acids were present in the rat or/and in the cat and human sequences. Changes are listed in Table 1. The M-CSF receptor is a transmembrane protein with intrinsic tyrosine kinase activity, shared by two catalytic domains (TK1 and TK2) separated by a kinase insert. The tyrosine residues in the intracellular domain are the target of phosphorylation either by the receptor itself or by other cellular kinases. Moreover, tyrosine phosphorylation modulates the binding of SH2 domain-containing proteins which regulate the signalling pathway (7). It was therefore noticeable that instead of a tyrosine residue at codon 660 within the TK1 domain we found an isoleucine residue, and conversely, instead of an isoleucine at codon 829 in TK2 a tyrosine residue. These differences were not due to a polymorphism between BALB/c mouse (3) and (C57Bl/6xBALB/c)Fl mouse (our sequence), as confirmed by RT-PCR done with synthetic primers encompassing codons 660 and 829.