Nature Immunology
3, 1200 - 1207 (2002)
Published online: 4 November 2002; | doi:10.1038/ni849
Identification of diversified genes that contain immunoglobulin-like variable regions in a protochordateJohn P. Cannon1, 2, Robert N. Haire3
& Gary W. Litman1, 2, 31
Immunology Program, H. Lee Moffitt Cancer Center and Research Institute, 12902 Magnolia Avenue, Tampa, FL 33612, USA. 2
All Children's Hospital, 801 Sixth Street South, St. Petersburg, FL 33701, USA. 3
Department of Pediatrics, University of South Florida College of Medicine, Children's Research Institute, 140 Seventh Avenue South, St. Petersburg, FL 33701, USA.
Correspondence should be addressed to Gary W. Litman litmang@allkids.orgThe evolutionary origin of adaptive immune receptors is not understood below the phylogenetic level of the jawed vertebrates. We describe here a strategy for the selective cloning of cDNAs encoding secreted or transmembrane proteins that uses a bacterial plasmid (Amptrap) with a defective  -lactamase gene. This method requires knowledge of only a single target motif that corresponds to as few as three amino acids; it was validated with major histocompatibility complex genes from a cartilaginous fish. Using this approach, we identified families of genes encoding secreted proteins with two diversified immunoglobulin-like variable (V) domains and a chitin-binding domain in amphioxus, a protochordate. Thus, multigenic families encoding diversified V regions exist in a species lacking an adaptive immune response.
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