TIBS 19 - APRIL 1994 THE PROTEIN TYROSINE phos- phatases (PTPs) are a growing family of enzymes that function, in concert with protein tyrosine kinases, to modulate the tyrosine phosphorylation of cellular proteins (for comprehensive reviews, see Refs 1 and 2). Of the 30 or so PTPs characterized to date, approximately a third are transmembrane, receptor-like molecules, while the majority are cytoplasmic proteins. With such a large number of potentially redundant en- zymes located intracellularly, the ac- tivity of the intracellular PTPs must be highly regulated to prevent spurious, nonspecific depbosphorylation of pro- teins. Regulation of the activity of such cellular enzymes can be accomplished in various ways: (I) by modulating the steady-state levels of the enzyme; (2) by substrate specificity; (3) by post- translational modification 0.e. phos- phorylation, proteolytic cleavage, etc.); and (4) by interaction with inhibitor/ activator molecules. We are just begin- ning to understand how these regulat- ory mechanisms might function to modu- late PTP activity. In particular, controlling substrate specificity by restricting sub- strate availability could be an import- ant means of regulating the intracellular PTPs. Sequence analyses of the PTPs have revealed an exquisite diversity of protein sequences outside the highly conserved catalytic domain. It has been suggested that these noncatalytic se- quences serve as 'zip codes' to 'ad- dress' the PTPs to specific comp,~rt ments within the ceil, thereby defining and restricting their substrate speci- ficity. Here we will discuss several examples of intracellulax PTPs that could he subjected to this mode of regulation. The PTP family The PTPs are multidomain proteins whose structural features separate the family into two main classes based on their transmembrane or intracellular location (Fig. 1). All PTPs, whether transmembrane or cytoplasmic, pos- sess at least one 230 amino acid cata- lytic domain containing a highly cnn- served active-site region with t!~e consensus motif [I/V]HCXAGXXR[S/I IG L. J. Maum and J. E. Dixon are at the Departmentof BiologicalChemistry, University of MichiganMedicalCenter, 5416 Medical Science I, Box 0606, Ann Arbor, M148109, USA. TALKING POINT Illlllll 'Zip codes' direct intracellular protein tyrosine phosphatases to the correct cellular 'address' [ Laura J. Mauro and Jack E. Dixon The transmembrane and intracellular protein tyrosine phosphatases (PTPs) play an essential role as signal transduction proteins involved in various cellular processes including division, proliferation and differen- tiation. As such, their activity must be strictly regulated to avoid nonspecific tyrosine dephosphorylation of cellular proteins. The intracellular P]Ps pos- sess a diversity of protein sequences outside the catalytic domain that appear to serve as 'zip codes' specifically 'addressing' these proteins to defined subceliular compartments. These localization strategies are pro- posed to function as a regulatory mechanism, defining the substrate specificity and function of the intracellular PTPs. (where X is any amino acid). This region is the signature sequence of most PTPs. Site-directed mutagenesis and trapping experiments have shown that the cysteinyi residue within this motif is essential for phosphatase ac- tivity and forms a thiol-phosphate intermediate during catalytic turnover1. Interestingly, this catalytic domain bears no resemblance to that of the serine/threonine phosphatases or o( the alkaline or acid phosphatases. Transmembrane PTPs possess an extracellular domain, a single trans- membrane domain and norma, dy two catalytic domains followed, by a short carboxy-terminal segment. At present, the human PTP[5 and tl'~e Drosophila DPTPI0D are the only transmembrane proteins that have a single catalytic domain. Unlike the cytoplasmic tail, the extracellular domains of these proteins are highly divergent, with small glycosyl- ated segments (human PTPa and e), tandem repeats of immunoglobulin-like and fibropectin type Ill domains similar to N.CAM molecules [leucocyte com- mon antigen related molecule OAR), PTP,~], or alternately spliced lengths of sequence containing N- and O.linked carbohydrates (CD45). These extracellu- lax features suggest that the activity of these PTPs might be modulated by ligands. Recently, it was shown that the homophilic binding of PTPp~ can mediate cell-cell aggregation 3,4. This suggests theft the extracellular domain of this PTP can serve as its own ligand and could potentially function to regu- late ceE-celi interactions. Thr: distinguishing feature of the intracellular PTPs is the diversity of de, mains flanking a single catalytic (iomain (Fig. 1). One of the principle functions of these flanking sequences, or zip codes, appears to be the targeting of the enzyme to specific intracellular locations. These include: (1) membrane- association domains, (2) nucleax- localization domains, (3) Src homology 2 (SH2) domains and (4) cytoskeletal- association domains. A schematic showing these intracellulax PTPs resid- ing at their proven or hypothesized subcellulax locations within a eukary- otic cell is shown in Fig. 2. We will dis- cuss each of the PTPs shown in the fol- lowing sections. Membrane and nuclear targeting sequences fol PTPs A number of intracellulax PTPs pos- sess unique sequences downstream of the catalytic domain, which might serve to regulate the targeting of the protein to a subcellular locale and, in some cases, also modulate phosphatase activity. Purification of PTP1B from the human placenta yielded both soluble and particulate forms of the enzyme, © 1994, Elsevier Science Ltd 096~- 0004,/94/$~Z00 '15~"