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

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Abstract
Botryllus schlosserl Is a colonlal tunicate that shared a common ancestor with the lineage leading to mammals about 450 million years ago, and flourishes today along the Caltfornia coast. Prior studles of Botryllus populations have demonstrated the presence of a co-dominantly expressed, highly polymorphic histocompatlblllty locus (FuIHC) controlling the acceptance (fusion) or rejection of new Individuals into a parablotlc colony. lntercolonlal blood cell contact, and recognition of setflnot self, precedes both fusion and rejection reactlons. 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 palm two or more polypeptides containing constant and variable regions. We have identified a disullfide-linked, heterodimeric (αβ) cell surface molecule in Botryllus with biochemical resemblance to mammalian lymphocyte antlgen receptors. Observed charge variants of constituent chalns of the tunicate protein described here do not correlate wtth FuMC allelic diversity. Both chains of thls heterdimer can be resolved into several lsofonns which are not based upon post-translational carbohydrate or phosphate additions. Comparisons of Iodinated tryptic peptides from two β chain Isomorphs reveal one distinct and several common peptides.