COOH-terminal sequence motifs target the T cell protein tyrosine phosphatase to the ER and nucleus

Abstract

The noncatalytic domain of the human T cell protein tyrosine phosphatase (TCPTP) is alternatively spliced to generate a 45-kD form, p45(TC), and a 48- kD form, p48(TC) (Champion-Arnaud et al., 1991; Mosinger et al., 1992). This manuscript concerns structural motifs in the noncatalytic segment of the enzyme responsible for targeting the two forms to different subcellular compartments. Endogenous and transiently expressed p48(TC) associates with the ER, as determined by sucrose gradient fractionation and indirect immunofluorescence, respectively. By contrast, p45(TC) localizes in the nucleus even though upon cell lysis it is not retained and fractionates with markers for soluble enzymes. Using fusion proteins consisting of β- galactosidase and COOH-terminal fragments of p48(TC), two motifs necessary for ER retention within a 70-residue targeting segment have been identified. These include the terminal 19 hydrophobic residues which comprise a potential membrane-spanning segment and residues 346-358 which encompass a cluster of basic amino acids that may represent another type of ER retention motif. The sequence RKRKR, which immediately precedes the splice junction, functions as a nuclear localization signal for p45(TC).