Interactions between membrane IgM and the cytoskeleton involve the cytoplasmic domain of the immunoglobulin receptor

Abstract

Cross-linking induced interactions between the membrane form of immunoglobulin (mIg) and the cytoskeletal matrix have been described by several groups. To date, the function of mIgM association with the cytoskeleton is not yet understood. Delineation of the molecular basis of these interactions will be instrumental in elucidating their function. We have previously shown that the Ig alpha/beta heterodimer is not required for ligand-induced mIgM binding to the cytoskeleton. In this study, we have investigated the role of other B cell-specific proteins in mediating these interactions. For this, we expressed mIgM in the non-hematopoietic human cervical carcinoma cell line HeLa S3 and verified the capacity of the surface-expressed IgM to interact with the cytoskeletal matrix upon cross-linking with anti-mu chain antibodies. We show here that only the mIgM molecule itself and no other B cell-specific protein(s) is required in mediating mIgM interactions with actin filaments. In an attempt to determine the cytoskeleton-binding site of mIgM we investigated the role of the cytoplasmic tail of mIgM (KVK) in binding the receptor to actin-based microfilaments. Using mutated forms of mIgM expressed in J558L cells, we show here that KVK plays a role in mediating these interactions. The absence of KVK did not, however, completely abrogate mIgM-cytoskeletal interactions, suggesting that there are additional molecular requirements for the ligand-induced mIgM binding to the cytoskeletal matrix.

The Actin Cytoskeleton is Required for the Trafficking of the B Cell Antigen Receptor to the Late Endosomes

“…Colocalization of BCRcontaining vesicles with the actin cytoskeleton may not necessarily indicate a direct interaction between the BCR and actin filaments. However, the data from this and previous studies are consistent with an interaction between the BCR and the actin cytoskeleton that is up-regulated by BCR activation (21)(22)(23)(24). This interaction may be either direct or indirect through adaptor proteins and actin-binding proteins recruited by the activated BCR (48,49).…”

SH3P7 Is a Cytoskeleton Adapter Protein and Is Coupled to Signal Transduction from Lymphocyte Antigen Receptors

“…A stimulation-dependent and -independent association of cytoskeletal components with both B-and T-cell antigen receptors has been reported previously (4,20,24,31). Antigen-mediated reorganization of the microtubule-organizing center in T cells and the F-actin assembly in B cells is ITAM dependent (6,19).…”

The pivotal position of the actin cytoskeleton in the initiation and regulation of B cell receptor activation

“…The initial studies in 1970 reported a redistribution of the actin motor protein myosin from the cell periphery to the BCR central cluster in response to BCR cross-linking by antibody [47] and an accumulation of the actin cytoskeleton at the plasma membrane of B cells in response to mitogens [48,49]. BCR cross-linking was later found to cause the association of surface BCRs and proximal signaling components with detergent insoluble cytoskeletal fraction of cells [50-53]. These implicate a physical and functional relationship of actin with surface BCRs and BCR signaling complexes.…”

“…An early observation that antigenic activation influences the physical relationship of both BCR and signaling molecules with the cytoskeleton [50-53] suggests another mechanism for actin to facilitate signaling: the actin cytoskeleton with actin adaptor proteins potentially serves as a scaffold to recruit and position signaling molecules physically close to the BCR. The reassembly of the actin cytoskeleton and the re-association of active ezrin with the lipid raft-associated-Csk binding protein after the initial detachment and disassembly can compartmentalize membrane proteins for receptor clustering and receptor signaling complex formation.…”