Characterization of a member of the immunoglobulin gene superfamily that possibly represents an additional class of growth factor receptor.

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Abstract
We have screened cDNA libraries prepared from embryonic chicken tissues to isolate additional genes encoding growth factor receptors. Nucleotide sequencing of a cDNA encoding a gene, which we have termed klg, revealed it to represent an additional member of the immunoglobulin gene superfamily, which also possesses extensive sequence similarity to the protein-tyrosine kinase growth factor receptor genes. The klg gene was shown to encode a 140-kDa glycoprotein. However, the sequence of the tyrosine kinase domain is unusual in that the aspartate residue located within the highly conserved Asp-Phe-Gly triplet is replaced by an alanine residue. The presence of this aspartate has previously been found to be essential for tyrosine kinase activity. Consistent with the replacement of this aspartate, we were unable to detect any evidence of an associated kinase activity with the klg-encoded protein. These observations raise the possibility that the klg gene product represents a newly discovered class of receptor that plays a role in signal attenuation rather than signal propagation.