p135 Src homology 2 domain-containing inositol 5'-phosphatase (SHIPβ) isoform can substitute for p145 SHIP in F(c)γRIIB1-mediated inhibitory signaling in B cells

Abstract

The inositol 5'-phosphatase, SHIP (also referred to as SHIP-1 or SHIPα), is expressed in all cells of the hematopoietic lineage. Depending on the cell type being investigated and the state of differentiation, SHIP isoforms of several different molecular masses (170, 160, 145, 135, 125, and 110 kDa) have been seen in immunoblots. However, the function of the individual isoforms and the effect of expressing multiple isoforms simultaneously are not understood. Some of these SHIP isoforms have recently been characterized at the level of primary sequence. In this report, we investigated the function of the recently characterized 135-kDa SHIP isoform (SHIPβ), which appears to possess the catalytic domain but lacks some of the protein-protein interaction motifs at the C terminus. By reconstituting SHIP-deficient DT40 B cells with either SHIPβ or the better-characterized p145 SHIPα, we addressed the function of SHIPβ in the complete absence of SHIPα. We observed that SHIPβ had enzymatic activity comparable with SHIPα and that SHIPβ was able to reconstitute F(c)γRIIB1-mediated inhibition of B cell receptor-induced signaling events such as calcium flux and Akt and mitogen-activated protein kinase activation. SHIPβ was readily phosphorylated in response to B cell receptor cross-linking with the inhibitory receptor F(c)γRIIB1 and SHIPβ also interacted with the adapter protein Shc. During these studies we also observed that the SHIPγ or SHIPβ interaction with Grb2 is not required for F(c)γRIIB1-mediated inhibition of calcium flux. These data suggest that SHIPβ, which is normally expressed in B cells along with SHIPα, functions comparably with SHIPα and that these two isoforms are not likely to be antagonistic in their function in vivo.