The CEA family: a system in transitional evolution?

The CEA family consists of two structurally and functionally distinct sub-groups; the group including CEA, NCA and CGM-6 which are cell surface-bound by phosphatidyl-inositol (PI) linkages, and the group of BGP splice variants which have trans-membrane and cytoplasmic domains. Although all CEA family members mediate intercellular adhesion in vitro, the PI-linked group show Ca++ and temperature independent adhesion whereas the BGP group show rapidly reversible Ca++ and temperature dependent adhesion. From the close alignment in cDNA nucleotide sequences between family members and between repeated domains in one family member, it is apparent that the CEA family is now rapidly evolving; in fact, analogs of only the trans-membrane BGP group have been found so far in the mouse. The addition of a new group of potent adhesion molecules to complex species at some time after the rodent radiation has strong evolutional implications, which are discussed.