Sorting signals in the MHC class II invariant chain cytoplasmic tail and transmembrane region determine trafficking to an endocytic processing compartment.

Abstract

Abstract. Targeting of MHC class II molecules to the endocytic compartment where they encounter processed antigen is determined by the invariant chain 0i). By analysis of Ii-transferrin receptor (TR) chimera trafficking, we have identified sorting signals in the Ii cytoplasmic tail and transmembrane region that mediate this process. Two non-tyrosine-based sorting signals in the Ii cytoplasmic tail were identified that mediate localization to plasma membrane clathrincoated pits and promote rapid endocytosis. Leu 7 and Ile s were required for the activity of the signal most distal to the cell membrane whereas pro15 Met16 L~ul7 were important for the membrane-proximal signal.The same or overlapping non-tyrosine-based sorting signals are essential for delivery of Ii-TR chimeras, either by an intracellular route or via the plasma membrane, to an endoeytic compartment where they are rapidly degraded. The Ii transmembrane region is also required for efficient delivery to this endocytic processing compartment and contains a signal distinct from the Ii cytoplasmic tail. More than 80 % of the Ii-TR chimera containing the Ii cytoplasmic tail and transmembrane region is delivered directly to the endocytic pathway by an intracellular route, implying that the Ii sorting signals are efficiently recognized by sorting machinery located in the trans-Golgi.

The chondroitin sulfate form of invariant chain trimerizes with conventional invariant chain and these complexes are rapidly transported from thetrans-Golgi network to the cell surface

“…Alternatively, some proteoglycans are degraded in endocytic compartments following internalization [35,36]. The cytosolic domain of Ii functions as an efficient internalization signal [20,37,38]. Therefore rapid endocytosis of Ii-CS from the cell surface and lysosomal degradation could account for the short half-life of Ii-CS.…”

Immune surveillance of intracellular pathogens via autophagy

“…13 MHC class II molecules migrate to these late endosomal compartments after cotranslational insertion into the ER, because they associate with the transmembrane protein invariant chain (I i ). I i not only blocks the peptide-binding groove of newly synthesized MHC class II molecules, 14 but also contains an endosomal targeting signal 15 and thus targets MHC class II molecules to late endosomes, where they meet peptides generated by lysosomal proteases. In this MHC class II loading compartment, lysosomal proteases also degrade the I i , and the remaining peptide (CLIP for class II-associated I i peptide) is exchanged for antigenic peptides with the help of the nonclassical MHC class II molecule HLA-DM.…”

Down-Regulation of Cell Surface Receptors Is Modulated by Polar Residues within the Transmembrane Domain

“…Mutants were prepared as described previously (Odorizzi et al, 1994) by the method of Kunkel (1985). The Ii TM 11-19 mutant was originally described in Kang et al (1998).…”

“…Human TR and Ii-TR mutants were expressed in CEFs as described previously (Odorizzi et al, 1994) by using the BH-RCAS expression vector. Metabolic labeling and Immunoprecipitation of CEFs expressing the various TR and Ii-TR constructs were performed as described previously (Odorizzi et al, 1994).…”

“…Perhaps the clearest example of this is the epidermal growth factor receptor that requires an active cytoplasmic kinase domain for entry into internal vesicles of the multivesicular body (Futter et al, 1993). Other late endosomal/lysosomal targeting signals have been identified in the cytoplasmic tails of P-selectin (Green et al, 1994), the mannose-6-phosphate receptor (Johnson and Kornfeld, 1992a,b), the major histocompatibility complex (MHC) class II invariant chain (Ii) (Bakke and Dobberstein, 1990;Pieters et al, 1993;Bremnes et al, 1994;Odorizzi et al, 1994;Pond et al, 1995;Kang et al, 1998), the T cell receptor CD3 ␥-chain (Letourneur and Klausner, 1992), lysosomal acid phosphatase (Peters et al, 1990), and lysosome-associated membrane glycoprotein (Williams and Fukuda, 1990), suggesting that entry into multivesicular bodies does not occur by default.…”