We and others have shown that newly synthesized class I heavy chains awaiting peptides in the ER are detectable in association with TAP only in cell lines that also express P2m This observation shows that class I heavy chains require the presence of β2m in order to associate with TAP. To determine whether β2m can associate with TAP in the absence of heavy chains, we tested the HLA-deficient cell line LCL 721.221. High levels of βjtn were found to be TAP associated in 721.221 cells, which indicates that βjin can bind TAP in the absence of HLA-A, B, and C. Indeed, expression of HLA-B27 by 721.221-B27 transfectants did not increase the amount of P2m associated with TAP. To investigate whether other molecules are required for $fn to associate with TAP, we analyzed the LCL 721.220 cell line. LCL 721.220 has been shown by others to lack an unknown factor (encoded by a gene linked to the MHC) that is necessary for normal class I assembly and surface expression. Interestingly, f>m was not associated with TAP in 721.220 cells. This finding suggests that the factor reported to be absent in 721.220 may be an intermediary between β2m and TAP. Our findings clearly indicate that the binding of β2m to TAP plays a key role in the assembly of class I MHC heterotrimers, and studies are underway to identify residues on β2ni that are needed for the interaction with TAP.
|State||Published - Dec 1 1996|