N-linked glycosylation selectively regulates autonomous precursor BCR function

Abstract

Functional μ-immunoglobulin heavy chains pair with VpreB and λ5 to form precursor B cell antigen receptors. Jumaa and colleagues show that glycosylation at asparagine 46, a unique modification specific to the μ-heavy chain, is required for the function of such receptors. Developing B cells express distinct classes of B cell antigen receptors (BCRs) that differ in their heavy chain (HC). Although only μHC is expressed in early stages, δHC-containing BCRs dominate on the surface of mature B cells. The reason for the tightly regulated expression of these receptors is poorly understood. Here we show that μHC was specifically required for precursor BCR (pre-BCR) function and that δHC was unable to form a functional pre-BCR. A conserved asparagine (N)-linked glycosylation site at position 46 (N46) in the first conserved domain of μHC was absolutely required for pre-BCR function, and swapping that domain with δHC resulted in a functional δHC-containing pre-BCR. When tested in the context of the BCR, μHC with a mutant N46 showed normal function, which indicated that N46-glycosylation is specifically required for pre-BCR function. Our results suggest an unexpected mode of pre-BCR function, in which binding of the surrogate light chain to N46 mediates autonomous crosslinking and, concomitantly, receptor formation.