Structural similarity between a primitive chordate membrane heterodimer and lymphocyte antigen receptors

Botryllus schlosseri is a colonial tunicate that shared a common ancestor with the lineage leading to mammals about 450 million years ago, and flourishes today along the California coast. Prior studies of Botryllus populations have demonstrated the presence of a co-dominantly expressed, highly polymorphic histocompatibility locus (Fu/HC) controlling the acceptance (fusion) or rejection of new individuals into a parabiotic colony. Intercolonial blood cell contact, and recognition of self/not self, precedes both fusion and rejection reactions. Efforts to understand the evolution of the immune system necessitate study of cell surface molecules involved in cell-cell recognition events in primitive species. In mammals, birds, amphibians, and fishes clonally distributed lymphocyte surface molecules that are responsible for antigen recognition (B cell immunoglobulins and T cell receptors) can be distinguished by the disulfide linkage that pairs two or more polypeptides containing constant and variable regions. We have identified a disulfide-linked, heterodimeric (alpha beta) cell surface molecule in Botryllus with biochemical resemblance to mammalian lymphocyte antigen receptors. Observed charge variants of constituent chains of the tunicate protein described here do not correlate with Fu/HC allelic diversity. Both chains of this heterodimer can be resolved into several isoforms which are not based upon post-translational carbohydrate or phosphate additions. Comparisons of iodinated tryptic peptides from two beta chain isomorphs reveal one distinct and several common peptides.