Association of the T-cell protein tyrosine phosphatase with nuclear import factor p97

Alternative splicing of the T-cell protein tyrosine phosphatase (TCPTP) transcript generates two forms of the enzyme that differ at their extreme C termini: a 48-kDa endoplasmic reticulum-associated form and a 45-kDa nuclear form. By affinity chromatography, using GST-TCPTP fusion proteins, we have isolated three cytoplasmic proteins of 120, 116, and 97 kDa that interact with TCPTP. The p120 protein associated with residues 377-415 from the C terminus of the 48-kDa form of TCPTP, whereas the recognition site for p97 and p116 was mapped to residues 350-381 encompassing the TCPTP nuclear localization sequence (NLS). The TCPTP NLS was shown to be bipartite, requiring basic residues 350-358 (basic cluster I) and 377-381 (basic cluster II), the sites of interaction with p97 and p116, for efficient nuclear translocation. The interaction between p97, p116, and the TCPTP NLS appeared unique in that these proteins did not form a stable interaction with the classical NLS of SV40 large T antigen or the standard bipartite NLS of nucleoplasmin. Sequence analysis of p97 identified it as the nuclear import factor p97 (importin-β), which is an essential component of the nuclear import machinery. In assays in vitro in permeabilized cells, p97 was necessary but not sufficient for optimal nuclear import of TCPTP. We found that TCPTP co-immunoprecipitated with the nuclear import factor p97 from cell lysates and that purified recombinant p97 and TCPTP interacted directly in vitro. These results indicate selectivity in the binding of p97 and p116 to the TCPTP NLS and suggest that p97 may mediate events that are distinct from the classical nuclear import process. Moreover, these results demonstrate that the C-terminal segment of TCPTP contains docking sites for interaction with proteins that may function to target the enzyme to defined intracellular locations and in the process regulate TCPTP function.