Evolution of the opossum MHC: Evidence for diverse alternative splice patterns and low polymorphism among class I genes.

Baker, M. L., Melman, S. D., Huntley, J., and Miller, R.D.

Abstract:
The opossum MHC shares a common organization with that of non-mammals while containing a diverse set of class I genes more typical of eutherian (placental) mammals. There are eleven class I loci in the opossum’s MHC region, seven of which are known to be transcribed. The previously described Modo-UA1 and Modo-UG display characteristics consistent with them being classical and non-classical class I genes respectively. Here we describe characteristics of the remaining five transcribed class I loci (Modo-UE, -UK, -UI, -UJ and -UM). All five genes have low or non-polymorphic peptide binding grooves, contain unpaired cysteines with the potential to result in homodimer formation and display genomic organizational features that would be unusual for classical class I loci. In addition, Modo-UJ and -UM were expressed in alternatively spliced mRNA forms, including a potentially soluble isoform of Modo-UJ. Thus, the opossum’s MHC region contains a single class I gene that is clearly classical and six other class I genes each with its own unique characteristics that likely perform roles other than or in addition to antigen presentation.