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 [3-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. p ROTEIN tyrosine phosphatases (PTPs) 1 exist as both intracellular and receptor-linked enzymes. All are related through a conserved catalytic domain which retains an essential cysteinyl residue surrounded by the PTP consensus sequence (I/V) HCXAGXXR (Sfr) G. IntraceUular PTPs possess segments outside their conserved catalytic domain that often bear distinct structural characteristics. SH2 (src homology 2) domains, PEST sequences, and sequences homologous to the retinaldehyde-binding protein or to cytoskeletal proteins that include band 4.1, ezrin, and talin have been described. These noncatalytic regions appear to have both a regulatory and localization function (Fischer et al., 1991; Charbonneau and Tonks, 1992). Many PTPs display broad and overlapping substrates specificities in vitro. Consequently, restricting the subcellular localization of the intracellular PTPs may play an important role in determining which substrates they may act upon.

The T cell protein tyrosine phosphatase (TCPTP) cDNA was initially obtained from a human peripheral T cell library and found to encode a ubiquitous 48-kD protein, p48 Tc (Cool et al., 1989). It has a COOH-terminal noncatalytic segment that is largely hydrophilic except for the last