Molecular biology of IL9 and IL9R

Both mouse and human IL-9 cDNAs contain a single long open reading frame of 432 nucleotides, which encodes a 144 amino acid polypeptide with an 18 amino acid signal sequence. They also contain four JV-linked glycosylation sites, which is consistent with the extensive size heterogeneity of IL-9 protein observed with IL-9-expressing cells. The overall homology between mouse and human IL-9 cDNAs is 67% at the nucleotide level and 56% at the amino acid level. The human IL-9 gene has been mapped to the long arm of human chromosome 5 while the mouse IL-9 gene has been localized to mouse chromosome 13. Both genes are 4 kb in length and consist of five exons and four introns. The 5'-flanking region of the IL-9 gene contains several potential transcriptional control elements. It has been shown recently that the cooperation of different transcriptional factors including NFkB, c-jun and a 35 kDa protein is essential for the constitutive and induced human IL-9 gene expression in human T lympho-tropic virus type I (HTLV-I) transformed T cells. Although no IL-9 message can be detected in freshly isolated human peripheral blood mononuclear cells, T cell mitogens such as phytohemagglutinin (PHA) or anti-CD3 induced substantial IL-9 expression that can be further enhanced by phorbol myristate acetate (PMA) or CD28. In the mouse system, IL-9 is mainly produced by TH2 clones in vitro, and in vivo IL-9 production is also associated with TH2-like T cell responses.

The mouse IL-9R encodes a 468 amino acid polypeptide with two potential N-linked glycosylation sites and six cysteine residues in the extracellular domain. The position of these cysteine residues, the presence of a WSEWE motif in the extracellular domain and the lack of consensus sequences for tyrosine or serine/threonine kinases in the intracellular domain suggest that IL-9R is a member of the hema-topoietin receptor superfamily. The human IL-9R encodes a 522 amino acid polypeptide with 53% homology with the mIL-9R. The extracellular region of human and mouse IL-9R is well conserved while the intracellular domain is quite different between the two. Despite the high sequence homology of ligand and receptor between the two species, mouse IL-9 interacts with mouse and human IL-9R while human IL-9 only targets human but not mouse cells. Mouse II.-9R gene is a single-copy gene composed of nine exons and eight introns, on the other hand, the human genome contains at least four IL-9R pseudogenes with 90% homology with the IL-9R gene. Some of the molecular characteristics of IL-9 and its receptor are summarized in Table 1.

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