InterPro

Glycoprotein U21 (gU21) binds to MHC class I molecules in the endoplasmic reticulum and targets them from the Golgi directly to the lysosomes. Once in the lysosomes both proteins are degraded. In consequence, surface class I molecules are down-regulated and infected cells are masked for immune recognition by cytotoxic T lymphocytes [PMID: 14617346, PMID: 17467766].

gU21 is a single-pass type I membrane protein located in the host cell endoplasmic reticulum membrane. The ER-lumenal domain associates with class I MHC molecules and is responsible for lysosomal sorting [PMID: 14617346].