SHIP-1 adapter functions mediate recruitment of FCRL1 to the BCR and inhibition of ERK

FCRL1 is a plasma membrane coreceptor on B cells that has been shown to potentiate B cell receptor (BCR)-driven calcium flux and negatively regulate ERK phosphorylation in a GRB2 and tyrosine-dependent manner. Of the proteins that associate with FCRL1, the recruitment of the inositol phosphatase SHIP-1 is GRB2 dependent, implicating SHIP-1 in FCRL1-mediated ERK regulation. Using immunoprecipitation and Western blotting, it was found that a proline-rich region in the C-terminus of SHIP-1, rather than its N-terminal SH2 domain, mediates recruitment of SHIP-1 to Y281 in FCRL1 in a manner dependent on GRB2. Interestingly, Y281 and GRB2 were also required for FCRL1 colocalization to the BCR. Translational fusions between tyrosine-mutated FCRL1 and the SH2 domain of SHIP-1 were sufficient to drive both colocalization of FCRL1 with the BCR as well as the ERK-inhibitory activity of FCRL1. Our data are consistent with a function for SHIP-1 as an adapter between FCRL1 and the BCR signalosome, and that this adapter function is responsible for BCR/FCRL1 colocalization after BCR stimulation and modulation of ERK signaling.