Mutation of a Single Conserved Residue in VH Complementarity-Determining Region 2 Results in a Severe Ig Secretion Defect

Abstract

During an immune response, somatic mutations are introduced into the VH and VL regions of Ig chains. The consequences of somatic mutation in highly conserved residues are poorly understood. Ile51 is present in 91% of murine VH complementarity-determining region 2 sequences, and we demonstrate that single Ile51→Arg or Lys substitutions in the PCG1-1 Ab are sufficient to severely reduce Ig secretion (1–3% of wild-type (WT) levels). Mutant H chains, expressed in the presence of excess L chain, associate with Ig binding protein (BiP) and GRP94 and fail to form HL and H2L assembly intermediates efficiently. The mutations do not irreversibly alter the VH domain as the small amount of mutant H chain, which assembles with L chain as H2L2, is secreted. The secreted mutant Ab binds phosphocholine-protein with avidity identical with that of WT Ab, suggesting that the combining site adopts a WT conformation. A computer-generated model of the PCG1-1 variable region fragment of Ig (Fv) indicates that Ile51 is buried between complementarity-determining region 2 and framework 3 and does not directly contact the L chain. Thus, the Ile51→Arg or Ile51→Lys mutations impair association with the PCG1-1 L chain via indirect interactions. These interactions are in part dependent on the nature of the L chain as the PCG1-1 VH single Ile51→Arg or Ile51→Lys mutants were partially rescued when expressed with the J558L λ1 L chain. These results represent the first demonstration that single somatic mutations in VH residues can impair Ig secretion and suggest one reason for the conservation of Ile51 in so many Ig VH.

Repertoire Shift in the Humoral Response to Phosphocholine-Keyhole Limpet Hemocyanin: VH Somatic Mutation in Germinal Center B Cells Impairs T15 Ig Function

“…This phenotype was consistent among multiple transfectants that each produced intracellular H and L chains but failed to readily form H 2 L 2 complexes (data not shown). In addition, mutant H chains displayed an association with molecular chaperones BiP and GRP94 implicating retention in the endoplasmic reticulum (data not shown), a result similar to our previous findings with Ab secretion mutants created by in vitro mutagenesis (27,57). The molecular basis of secretion loss is difficult to dissect for mutants V H 1-6 and V H 1-8 because they contain multiple mutations.…”

“…T15 Abs are reported to undergo little affinity maturation (23,55) and in vitro studies have demonstrated that random mutations introduced into T15V H CDR2 and framework region 2 frequently resulted in loss of binding or failure to be secreted (25,26,56,57). However, group II V regions appear to be equally susceptible to harmful mutations (54,56,58).…”

Aggregates, Crystals, Gels, and Amyloids: Intracellular and Extracellular Phenotypes at the Crossroads of Immunoglobulin Physicochemical Property and Cell Physiology

“…However, it was an important example where deleterious secretion-impairing mutations on one variable domain can be overcome by compensatory mutations in the variable domain of the other subunit. Various groups have reported different point mutations that abolish efficient Ig assembly and secretion to date [75–78]. It remains to be experimentally shown, but still is an attractive possibility to consider that somatic hypermutations are able to restore those secretion defects by introducing compensatory mutations on the other subunit.…”

Recovering Antibody Secretion Using a Hapten Ligand as a Chemical Chaperone

“…Previously, we demonstrated that 16 mutants in the H chain framework region 2 and CDR2 region of the T15 and PCG1-1 anti-phenylphosphocholine antibodies have severe defects in secretion (6,7). The molecular basis of the secretion defect in several mutants was traced to a single substitution (Ile 51 to Arg or Lys) in the H chain of the PCG1-1 antibody (8). Mutant H chains co-immunoprecipitate with chaperones BiP and GRP94 and remain endoglycosidase H-sensitive, consistent with retention in the endoplasmic reticulum (ER).…”

“…Mutant H chains co-immunoprecipitate with chaperones BiP and GRP94 and remain endoglycosidase H-sensitive, consistent with retention in the endoplasmic reticulum (ER). Mutant H chains rapidly assemble into intracellular H 2 intermediates but exhibit impaired L chain association and undergo intracellular degradation (8).…”