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

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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 Igα/β 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‐μ 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.