RESUMEN
The nucleotide sequence of chicken invariant chain (Ii) was determined, and the amino acid sequence similarity with human Ii is 61%. Certain regions important for the biological function of human Ii are highly conserved between chicken and mammals. The cytoplasmic tail of chicken Ii fused to the plasma membrane reporter molecule neuraminidase relocated the protein to endosomes. Moreover, like the mammalian orthologs, the cytoplasmic tail was found to contain two independent leucine-based endosomal sorting signals. Chicken Ii was found to interact with human Ii and crosslinking studies also indicate that chicken Ii assembles as a trimer. The chicken Ii can furthermore bind the human MHC class II (HLA-DR1). Many of the functional properties between the chicken Ii and its mammalian orthologs are thus maintained in spite of their sequence differences.
Asunto(s)
Antígenos de Diferenciación de Linfocitos B/genética , Antígenos de Diferenciación de Linfocitos B/inmunología , Endocitosis/inmunología , Antígenos de Histocompatibilidad Clase II/genética , Antígenos de Histocompatibilidad Clase II/inmunología , Transducción de Señal/inmunología , Animales , Antígenos de Diferenciación de Linfocitos B/análisis , Membrana Celular/química , Membrana Celular/inmunología , Pollos , Clonación Molecular , ADN Complementario , Endosomas/química , Endosomas/inmunología , Expresión Génica/inmunología , Antígenos de Histocompatibilidad Clase II/análisis , Humanos , Leucina , Mamíferos , Datos de Secuencia Molecular , Señales de Clasificación de Proteína/genética , Señales de Clasificación de Proteína/inmunología , Proteínas Recombinantes de Fusión/genética , Proteínas Recombinantes de Fusión/inmunología , Homología de Secuencia de Aminoácido , TransfecciónRESUMEN
Invariant chain (Ii) is a transmembrane type II protein that forms a complex with the major histocompatibility complex (MHC) class II molecules in the endoplasmic reticulum (ER). The membrane proximal luminal region of Ii is responsible for the non-covalent association with MHC class II molecules. Chemical cross-linking in COS cells was used to study the effect of luminal and cytoplasmic deletions on trimerization of Ii. We demonstrate that trimerization of Ii is independent of the cytosolic tail of Ii, whereas residues 162-191 (the sequence encoded by exon 6) in the luminal part of Ii are essential for trimer formation. Immunofluorescence studies of the transfected luminal deletion constructs show that the amino acids encoded by exon 6 of Ii are also essential for the induction of large endosomal vesicles. The data suggest that Ii must be in a trimeric form to modify the endosomal pathway.
Asunto(s)
Antígenos CD/metabolismo , Antígenos de Diferenciación de Linfocitos B/metabolismo , Antígenos de Neoplasias/metabolismo , Endosomas/metabolismo , Exones , Antígenos de Histocompatibilidad Clase II/metabolismo , Proteínas de Neoplasias/metabolismo , Secuencia de Aminoácidos , Aminoácidos/análisis , Aminoácidos/genética , Animales , Células COS , Humanos , Ratones , Datos de Secuencia Molecular , Conformación Proteica , Eliminación de Secuencia , Relación Estructura-Actividad , TransfecciónRESUMEN
Invariant chain (Ii) is a transmembrane protein that associates with the MHC class II molecules in the endoplasmic reticulum. Two regions of the 30 residue cytoplasmic tail of Ii contain sorting information able to direct Ii to the endocytic pathway. The full-length cytoplasmic tail of Ii and the two tail regions were fused to neuraminidase (NA) forming chimeric proteins (INA). Ii is known to form trimers and when INA was transfected into COS cells it assembled as a tetramer like NA. The INA molecules were targeted to the endosomal pathway and cotransfection with Ii showed that both molecules appeared in the same vesicles. By labelling the INA fusion proteins with iodinated antibody it was found that molecules with either endocytosis signal were expressed at the plasma membrane and internalized rapidly. Point mutations revealed that an LI motif within the first region of the cytoplasmic tail and an ML motif in the second region were essential for efficient internalization. The region containing the LI motif is required for Ii to induce large endosomes but a functional LI internalization motif was not fundamental for this property. The cytoplasmic tail of Ii is essential for efficient targeting of the class II molecules to endosomes and the dual LI and ML motif may thus be responsible for directing these molecules to the endosomal pathway, possibly via the plasma membrane.