cDNA and Gene Analyses Imply a Novel Structure for a Rat Carcinoembryonic Antigen-related Protein

The gene encoding the human tumor marker carcinoembryonic antigen
(CEA) belongs to a gene family which can be subdivided into the CEA
and the pregnancy-specific glycoprotein subgroups. The
corresponding proteins are members of the immunoglobulin
superfamily, characterized through the presence of one IgV-like
domain and a varying number of IgC-like domains. Since the function
of the CEA family is not well understood, we decided to establish
an animal model in the rat to study its tissue- specific and
developmental stage-dependent expression. To this end, we have
screened an 18-day rat placenta cDNA library with a recently
isolated fragment of a rat CEA-related gene. Two overlapping clones
containing the complete coding region for a putative 709 amino acid
protein (rnCGM1; Mr = 78,310) have been characterized. In contrast
to all members of the human CEA family, this rat CEA-related
protein consists of five IgV-like domains and only one IgC-like
domain. This novel structure, which has been confirmed at the
genomic level might have important functional implications. Due to
the rapid evolutionary divergence of the rat and human CEA gene
families it is not possible to assign rnCGM1 to its human
counterpart. However, the predominant expression of the rnCGM1 gene
in the placenta suggests that it could be analogous to one of the
human pregnancy-specific glycoprotein genes.