TAPP1 and TAPP2 Are Targets of Phosphatidylinositol 3-Kinase Signaling in B Cells: Sustained Plasma Membrane Recruitment Triggered by the B-Cell Antigen Receptor

Abstract

We report the characterization of two signal transduction proteins related to Bam32, known as TAPP1 and TAPP2. Bam32, TAPP1, and TAPP2 share several characteristics, including small size (32 to 47 kDa), lack of enzymatic domains, high conservation between humans and mice, and the presence of pleckstrin homology (PH) domains near their C termini which contain the 3-phosphoinositide-binding motif. Unlike Bam32, the N-terminal regions of TAPP1 and TAPP2 contain a second PH domain. TAPP1 and TAPP2 transcripts are expressed in a variety of tissues including lymphoid tissues. Using live-cell imaging, we demonstrate that TAPP1 and TAPP2 are recruited to the plasma membrane of BJAB human B-lymphoma cells upon activation through the B-cell antigen receptor (BCR). The C-terminal PH domain is necessary and sufficient for BCR-induced membrane recruitment of both TAPP1 and TAPP2. Blockade of phosphatidylinositol 3-kinase (PI3K) activity completely abolished BCR-induced recruitment of TAPP1 and TAPP2, while expression of active PI3K is sufficient to drive constitutive membrane localization of TAPP1 and TAPP2. TAPP1 and TAPP2 preferentially accumulate within ruffled, F-actin-rich areas of plasma membrane, suggesting a potential role in PI3K-driven cytoskeletal reorganization. Like Bam32, BCR-driven TAPP1 and TAPP2 recruitment is a relatively slow and sustained response, in contrast to Btk recruitment and Ca²⁺ mobilization responses, which are rapid and transient. Consistent with recent studies indicating that Bam32, TAPP1, and TAPP2 can bind to PI(3,4)P₂, we find that membrane recruitment correlates well with production of PI(3,4)P₂ but not with that of PI(3,4,5)P₃. Our results indicate that TAPP1 and TAPP2 are direct targets of PI3K signaling that are recruited into plasma membranes with distinctive delayed kinetics and accumulate within F-actin-rich membrane ruffles. We postulate that the TAPPs function to orchestrate cellular responses during the sustained phase of signaling.

We thank Edward Clark for supporting the early stages of this work and for critical reading of the manuscript. We thank Tomas Balla and Doreen Cantrell for providing plasmid constructs and John Rutherford for help with confocal microscopy.

This work was supported by grants from the Canadian Institutes of Health Research (to A.J.M. and V.D.), Manitoba Health Research Council (to A.J.M.), and NIH (AI44257). A.J.M. is a CIHR New Investigator. V.D. is a CIHR/BCLA Scientist and a Michael Smith Foundation for Health Research Senior Scholar.