MOLECULAR-BASIS OF THE MEMBRANE-ANCHORED AND 2 SOLUBLE ISOFORMS OF THE HUMAN INTERLEUKIN-5 RECEPTOR ALPHA-SUBUNIT

By use of a 3' extension PCR strategy, cDNA clones were isolated spanning the transmembrane region and a complete cytoplasmic domain of the human interleukin 5 receptor alpha-subunit (hIL5R-alpha). These cDNAs differ from previously isolated clones encoding a soluble hIL5R-alpha form by a sequence switch at position 1243. When expressed in COS-1 cells, only low-affinity binding of I-125-labeled human interleukin 5 was observed. Coexpression of the hIL5R-beta chain led to a 2-fold increase in binding affinity. In addition, this same cloning strategy allowed us to identify a putative second soluble isoform of hIL5R-alpha. Genomic data revealed that the two soluble variants arise from either a "normal" splicing event or from the absence of splicing, whereas synthesis of the membrane-anchored form requires alternative splicing.