Identification of serine residues in the Connexin43 carboxyl tail important for BCR-mediated spreading of B-lymphocytes

B-lymphocytes recognize antigen via B cell antigen receptors (BCR). This binding induces signaling leading to B cell activation, proliferation and differentiation. Early events of BCR signaling include reorganization of actin, and membrane spreading for increased antigen gathering. We have previously shown that the gap junction protein connexin43 (Cx43) is phosphorylated upon BCR signaling, and its carboxyl tail (CT) is important for BCR-mediated spreading. Here, specific serines in the Cx43 CT that are phosphorylated following BCR stimulation were identified. A chimeric protein containing the extracellular and transmembrane domains of CD8 fused to the Cx43 CT was sufficient to support cell spreading. Cx43 CT truncations showed that the region between amino acids 246-307 was necessary for B cell spreading. Site-specific serine to alanine mutations, S255A, S262A, S279A and S282A, resulted in differential effects on both BCR signaling, and BCR-mediated spreading. These serines can serve as potential binding sites for actin remodeling mediators and/or BCR signaling effectors; therefore, our results may reflect unique roles for each of these serines in terms of linking the Cx43 CT to actin remodeling.