Translational diffusion of class II major histocompatibility complex molecules is constrained by their cytoplasmic domains

Site-directed mutagenesis in vitro was used to introduce stop codons in the genomic DNA of the α and β chains of the murine class II major histocompatibility complex antigen, I-A(k). Mutated DNA was transfected into B lymphoma cells that were then selected by neomycin resistance and for their ability to express I-A(k) molecules on their plasma membrane. The translational diffusion coefficient (D(lat)) of I-A(k) molecules composed of a wild-type β chain paired with an α chain missing either 6 or 12 amino acids from the cytoplasmic domain is on the average threefold higher than the D(lat) of wild-type I-A(k) molecules as measured by fluorescence photobleaching and recovery. The removal of 12 amino acids from the cytoplasmic domain of the β chain did not change the D(lat) value from that of wild-type I-A(k) if the truncated β chain was paired with a wild-type α chain. Removing all amino acids of the cytoplasmic domains of both the α and β chains resulted in a 10-fold increase in the D(lat), the higest value for any of the truncated I-A(k) molecules tested. These data indicate that the carboxy-terminal six amino acids of the cytoplasmic domain of the α chain and the six plasma membrane-proximal amino acids of the β chain are important in constraining the translational diffusion of I-A(k) molecules in the plasma membrane.