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Molecular cloning and site-directed mutagenesis of leucine-based sorting motifs of the porcine invariant chain

Author(s): F.Z. Xu, Y. Dai, L.M. Li, X.L. Ding and W.Y. Yu

Invariant chain (Ii) is a transmembrane protein that associates with MHC class II molecules in the endoplasmic reticulum. The cytoplasmic tail of Ii contains two leucine residues able to direct Ii to the endocytic pathway. We obtained the pig Ii gene by RT-PCR. Mutated Ii was prepared via site directed mutagenesis by the PCR Megaprimer method to study the effect of the two leucines on the localization of pig Ii. These mutated fragments were ligated to the vector pmCherry-C1. The recombinant plasmids were transiently transfected into COS-7 cells with LipofectamineTM 2000. Fluorescence of fusion proteins (mCherry-Ii) was observed with a fluorescent microscope. Amino acid sequence alignment showed that pig Ii has domains similar to those seen in other mammalian Ii, including the cytoplasmic, transmembrane, class II-associated Ii-derived peptide, and trimerization domains. Based on observations with the fluorescent microscope, we found that two leucine-based motifs are required for pig Ii intracellular localization, and that both motifs independently mediate this function in Ii.