CDNA Cloning And Analysis Of Molecular Structure And Function Of H. Molitrix Invariant Chain

Invariant chain (Ii) is a nonpolymorphic type Ⅱtransmembrane glycoprotein, whichmainly expresses on the surface of B cells, macrophage cells, dendritic cells and thymicepithelial cells etc. As a chaperone of MHCⅡ molecule, Ii mainly has the followingfunctions:(1) It can facilitate the proper fold of the MHC Ⅱheterodimeric and egress fromthe endoplasmic reticulum(ER);(2) It can provide a targeting signal for endosomal/lysosomal compartments;(3) It prevents peptide from associating prematurely withMHC Ⅱm olecule. So Ii plays a key role in MHCⅡ molecular presenting exogenous antigento CD4+T lymphocytes and has an important regulated significance on immune reaction.It was reported the DNA vaccine containing Ii gene could enhance the immune response.Invariant chain has an important function for MHC class II molecular in presentingantigen peptides to T cells. Meanwhile, Ii is as a specific MHC II chaperone, playing animportant role not only in antigen presentation but also as a crucial receptor. There havebeen more studies about mammal and bird Ii, but it is very insufficient about fish Ii. Toknow the structure and function of fish Ii, we cloned and sequenced first time Silver carp(Hypophthalmichthys molitrix) Ii gene from their spleen cells by RT-PCR and RACE. Thefull length Ii-cDNA was1136bp, its open reading frame (ORF) was699bp encoding232amino acids. A sequence alignment showed that the Ii had the four structure domains asother species, including transmembrane region, cytoplasmic region, endoplasmic reticulumregion(clip and Trim) and a thyroglobulin (Tg) type-I domain. The Ii gene and itsfunctional segments were cloned into an eukaryocyte expression vector pEGFP-C1respectively, and further these reconstructed vectors were transfected into eukaryoticexpression cell COS7. The results showed that Silver carp Ii was located on plasmamembrane as a transmembrane glycoprotein. The deduced amino acid sequences showedthat the Silver carp had a similarity of78%to Zebrafish and had a lower similarity of30%~40%to human and other mammals. The Ii3D molecular structure of different specieswas simulated and constructed. The comparison of them revealed a high similarity in thestructure and functions of among Iis of Silver carp, mouse, human and chicken, especiallysome amino acid residues at the important sites. These results demonstrated that Ii as animportant immune molecule has a high degree of homology in genetics and a similarity instructure and function among different animals. The study of fish Ii has made substantialbasis for the study of relation between Ii and MHC molecule.