engleski

Endocytosis and intracellular trafficking of MHC class I molecules

The total number of cell surface glycoprotein molecules at the plasma membrane results from a balance between their constitutive internalization, as a net result of constitutive endocytosis and endocytic recycling, and their egress to the cell surface from intracellular pools and/or biosynthetic pathway. MHC class I molecules are cell surface glycoproteins that are present within the cell in basically two forms: full, complexes of heavy chain (HC), β -2-microglobulin (β 2m) and peptide, and empty, HCs devoid of peptide and β 2m. In this study we have compared spontaneous internalization, recycling, endocytic trafficking and intracellular localization of murine (Kd, Dd, full Ld and empty Ld) and human (full and empty HLA, HLA-B7, HLA-Cw6 and HLA-G) MHC class I alleles. MHC endocytic pathways and their intracellular distribution was compared with the transferrin receptor pathway (clathrin dependent) and cholera-toxin pathway (lipid raft dependent). Intracellular localization was determined by confocal microscopy using a panel of endosomal markers: GM1 and caveolin for lipid raft rich vesicles, Rab 4, Rab5a, Rab5b and Rab11 Rab22 and EEA1 for classical endosomal compartments, calreticulin for endoplasmic reticulum, Lamp1 and Lamp2 for late endosomes, and a panel of Golgi markers and markers of the secretors pathways. MHC alleles differed regarding their cell surface stability, kinetics, and in the way of internalization and degradation. Full MHC molecules internalized via the bulk pathway and crossroad the transferrin pathway and cholera-toxin pathway. Empty MHC molecules were sorted into the lipid rafts at the cell surface and internalized by the lipid-raft dependent endocytosis. Full and empty MHC molecules separated into distinct intracellular compartments, indicating for their segregation into two intracellular pools. A majority of internalized full MHC molecules recycled back to the cell surface through the peripheral and perinuclear recycling compartments, whereas empty MHC molecules did not recycle through these recycling compartments, but rather accumulated in a large vesiculo-tubular compartments prior recycling or degradation. Misfolded MHC molecules were sorted into the lipid-rafts, used the same endocytic route as empty molecules and accumulated in the same vesiculo-tubular structures. The empty MHC molecule retention compartment overlapped neither with transferrin receptor trafficking compartments (early endosomes, sorting endosomes and recycling endosomes), nor with late endosomes and GM1+ compartmets, indicating for separate sleeve of intracellular trafficking of misfolded proteins.

MHC-I molecules; endocytosis; vesicular transport

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