A MUTATION OF THE MU-TRANSMEMBRANE THAT DISRUPTS ENDOPLASMIC-RETICULUM RETENTION - EFFECTS ON ASSOCIATION WITH ACCESSORY PROTEINS AND SIGNAL-TRANSDUCTION

The mu heavy chain has an unusually high content of hydroxyl-containin g amino acids in its membrane-spanning region. We have examined the in volvement of two of these hydrophilic residues in endoplasmic reticulu m (ER) retention, interactions with Ig-alpha/Ig-beta, and transmembran e signaling. Neighboring tyrosine and serine residues were mutated to either phenylalanine and alanine (mutant YS/FA) or valine and valine ( mutant YS/VV). Membrane Ig (mIgM) molecules containing these mutant mu chains were expressed on the surface of transfected B lymphoma cells. Anti-Ig-induced signaling by the YS/FA mutant mIgM was equivalent to wild-type (wt) mIgM, whereas signaling by the YS/VV mutant mIgM was no tably diminished. Association between mutant YS/VV mIgM and Ig-alpha/I g-beta was detectable but reduced in comparison to YS/FA or wt mIgM. S ignaling by YS/VV mutant mIgM appeared to involve Ig-alpha/Ig-beta, be cause these proteins were tyrosine phosphorylated on receptor cross-li nking. When YS/VV and wt mu chains were cotransfected with light chain s into nonlymphoid cells, mutant mIgM was expressed at the cell surfac e in the absence of Ig-alpha/Ig-beta, whereas wt mIgM was not. These d ata suggest that the mutated residues contribute to ER retention and d irectly or indirectly to association with Ig-alpha/Ig-beta. Moreover, ER retention can be disrupted without preventing functional associatio n with Ig-alpha/Ig-beta. In addition, these data indicate that the hyd roxyl groups of the mutated residues are not required for functional a ssociation between mu and Ig-alpha/Ig-beta because their removal did n ot reduce the ability of the YS/FA mutant mIgM to associate with acces sory proteins or to participate in signal transduction.