Abstract
The carcinoembryonic antigen (CEA) gene family belongs to the immunoglobulin (Ig) superfamily and codes for a vast number of glycoproteins that differ greatly both in amino acid composition and function. The CEA family is divided into two groups, the carcinoembryonic antigen-related cell adhesion molecules (CEACAMs) and the pregnancy-specific glycoproteins. The CEA family members are implicated in pleiotropic (patho)physiological functions including cell–cell adhesion, pregnancy, immunity, neovascularization, regulation of insulin homeostasis, and carcinogenesis. In general, the CEA-encoded proteins are composed of an extracellular region with Ig variable and constant-like domains and a cytoplasmic region containing signaling motifs. Of particular interest, the well-studied human and mouse CEA genes are arranged in clusters in a single chromosome. Taking into account this characteristic, we made an effort to reconstruct the evolutionary history of the CEA gene family. Toward this end, the publicly available genomes were searched extensively for CEA homologs. The domain organization of the retrieved protein sequences was analyzed, and, subsequently, comprehensive phylogenetic analyses of the entire length CEA homologous proteins were performed. A series of evolutionarily conserved amino acid residues, functionally important, were identified. The relative positioning of these residues on the modeled tertiary structure of novel CEA protein domains revealed that they are, also, spatially conserved. Furthermore, the chromosomal arrangement of CEA genes was examined, and it was found that the CEA genes are preserved in terms of position, transcriptional orientation, and number in all species under investigation.
Highlights
The carcinoembryonic antigen (CEA) gene family, which belongs to the immunoglobulin (Ig) gene superfamily, comprised an exceptionally diverse array of highly glycosylated glycoproteins (Paxton et al 1987; Zhou et al 2001)
The CEA family is divided into two groups, the carcinoembryonic antigen-related cell adhesion molecules (CEACAMs) and the pregnancy-specific glycoproteins
207 CEA protein-encoding genes, 13 pseudogenes, and 1 expressed sequence tag (EST) sequence were identified in the genomes of 20 species representing diverse eukaryotic taxonomic divisions rimates (87), rodentia (47), perissodactyla (9), cetartiodactyla (6), carnivore (10), afrotheria (5), xenarthra (7), metatheria (15), proteotheria (5), sauria (2), amphibia (13) and teleosts (14), and chondrichthyes (1)
Summary
The carcinoembryonic antigen (CEA) gene family, which belongs to the immunoglobulin (Ig) gene superfamily, comprised an exceptionally diverse array of highly glycosylated glycoproteins (Paxton et al 1987; Zhou et al 2001). In humans, based on our current phylogenetic analysis, the CEA family consists of 35 genes, 21 of which are protein coding, arranged in contiguous clusters in chromosome 19 in the region 19q13.2-19q13.4 (Hammarstrom 1999). All currently reported CEA-encoded proteins consist of at least one Ig variable (IgV)-like domain, followed by a varying number of Ig constant (IgC)-like domains (Brummendorf and Rathjen 1995). The IgV-like domain, approximately 110 amino acids long, contains a conserved basic (arginine) and an acidic (aspartate) amino acid, which are proposed to stabilize the Ig-like fold via an intradomain salt bridge. The IgC-like domain, contains two conserved cysteine residues, that occupy the corresponding positions of arginine and aspartate, stabilize the Ig-like conformation by forming a disulphide bridge (Williams and Barclay 1988; Bork et al 1994)
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