Point mutations define a mIgM transmembrane region motif that determines intersubunit signal transduction in the antigen receptor.
Ag binding to the membrane Ig (mIg) substructure of the B cell Ag receptor leads to activation of cytoplasmic effector molecules including blk, fyn, lyn, and/or lck tyrosine kinases that are associated with receptor's dimeric Ig-alpha/Ig-beta transducer substructure. The structural basis of the apparent intermolecular transmission of this information within the receptor complex is unknown. Here we report that conservative point mutation of a sequence, S584-K597, at the cytoplasmic end of the predicted transmembrane spanning domain of the mIgM heavy chain (mu) ablates Ag-activated signal transduction, while having no detectable effect on association of mIgM with Ig-alpha/Ig-beta heterodimers. Specifically, mutation of serine584 to alanine, tyrosine587 to phenylalanine, threonine592 to valine, or lysine597 to isoleucine completely abrogated Ag-induced signal transduction leading to protein tyrosine phosphorylation and Ca2+ mobilization. Interestingly, mutants in the more peripheral of these residues, serine584 to alanine and lysine597 to isoleucine, remained responsive to a monoclonal antireceptor Ab (b-7-6) and all mutants remained responsive to polyclonal antireceptor Ab. These data implicate the polar sequence, -Y587STTVT592-, in transfer of information from ligand binding to transducer substructures within this heterooligomeric receptor complex. They further indicate that receptor activation by ligands that bind with high affinity and/or to constant region mIg epitopes is less dependent on the integrity of this motif.
Pleiman
C M
CM
Department of Pediatrics, National Jewish Center for Immunology and Respiratory Medicine, Denver, CO 80206.
Chien
N C
NC
Cambier
J C
JC
D000595:Amino Acid Sequence; D001483:Base Sequence; D002118:Calcium / Q000378:metabolism; D007075:Immunoglobulin M / Q000502:physiology*; D008969:Molecular Sequence Data; D010766:Phosphorylation; D017354:Point Mutation; D011947:Receptors, Antigen, B-Cell / Q000502:physiology*; D015398:Signal Transduction; D014443:Tyrosine / Q000378:metabolism
D007075:Immunoglobulin M; D011947:Receptors, Antigen, B-Cell; D014443:Tyrosine; D002118:Calcium