PEST Control”: Regulation of Molecular Barcodes by Tyrosine Phosphatases Cell Research (2014) 24:1027-1028

“PEST control”: regulation of molecular barcodes by tyrosine phosphatases Cell Research (2014) 24:1027-1028. doi:10.1038/cr.2014.100; published online 1 August 2014

The emerging concept of “molecu- phosphorylation activates MAPK sig- phorylation- and ubiquitination-based lar barcodes” refers to the dynamic naling [6]. In an elegant series of experi- barcoding mechanism of regulation of combination of post-translational ments, and through co-crystallization the intracellular fate of an important modifications, often of different nature of PTPN18 with peptides derived from oncogene, this study also introduces (e.g., phosphorylation and ubiqui- phosphorylated HER2, Wang et al. two novel concepts in the biochem- tination) that gives rise to multiple identified “selective loss-of-function” istry and cellular biology of tyrosine forms of a protein which can relay point mutations of PTPN18 that abol- phosphatases. First, by providing an distinct signals throughout a cell. In a ish the ability of the phosphatase to example of structure-based design of recent Cell Research paper by Wang dephosphorylate each one of the three mutants able to selectively abolish the et al., the authors report that a PEST above-mentioned HER2 phosphoryla- capacity of dephosphorylating single domain-containing tyrosine phospha- tion sites. The authors then proceeded to sites, this study provides rigorous sup- tase, PTPN18, is able to regulate both express these selective loss-of-function port to the now widely-accepted notion phosphorylation and ubiquitination PTPN18 mutants in breast cancer cells that phosphatases are not promiscuous of the HER2 oncogene, barcoding in order to assess the cellular pheno- enzymes. Rather, they recognize dif- HER2 for increased proteasomal deg- types elicited by hyperphosphorylation ferent sites in unique ways that depend radation rather than for intracellular of each PTPN18-target site. When they on structural determinants outside trafficking. expressed the PTPN18 P109A mutant, the active site. While this concept is HER2 is a prominent oncogene which is unable to dephosphorylate not new, and has led to the successful overexpressed in breast cancer and the pY1112 site, the authors indeed ob- design of bi-dentate and multi-dentate several other solid cancers, and is a served an increase in phosphorylation- tyrosine phosphatase inhibitors with drug target of Trastuzumab, a mono- dependent HER2 lysosomal trafficking. enhanced enzyme selectivity features clonal antibody that is FDA-approved However, when they subjected cells [7], it is the first time that it is extended for treatment of HER2+ breast cancers to complete knockdown of PTPN18, to selective targeting of phosphorylation [1]. HER2 undergoes phosphorylation there was a paradoxical increase in sites within the same molecule. The on several tyrosine residues, each one HER2 expression. By following this Wang et al. approach to phosphatase regulating specific intracellular func- unexpected observation, they identified selectivity through structural biology tions. In a recent paper published in a new mechanism of PTPN18-mediated paves the way to the rational design of Cell Research, Wang et al. [2] first regulation of HER2 half-life. They compounds for selective inhibition of defined which phosphorylation sites showed that PTPN18 promotes HER2 single site dephosphorylation, which on HER2 are regulated by protein tyro- ubiquitination and proteasomal degra- would be desirable for biology studies sine phosphatase non-receptor type 18 dation through a physical interaction and signaling therapies [8]. (PTPN18). They identified that three between the PTPN18 PEST domain and The second innovative aspect of HER2 C-terminal tyrosines are de- HER2, which enables the recruitment the report by Wang et al. [2] is the phosphorylated by PTPN18: Y1112, Y1196 of the E3 ubiquitin ligase β-Trcp to the demonstration that two domains of a and Y1248. Previous reports have shown HER2 complex. The authors also pro- tyrosine phosphatase mediate different that phosphorylation of Y1112 leads to vided indirect evidence suggesting that post-translational modifications of the enhanced Cbl-mediated K63-linked phosphorylation of serine residues in the same target molecule. Tyrosine phos- ubiquitination and lysosomal targeting PEST domain of PTPN18 is necessary phatases such as PTPN11 (SHP-2) can of HER2 [3], while Y1196 phosphoryla- to promote ubiquitination and degrada- regulate signaling transduction through tion leads to increased cell motility and tion of HER2. catalytic-dependent and -independent decreased apoptosis [4, 5], and Y1248 In addition to unraveling a new phos- manners [9]. We also recently reported npg 1028 by tyrosine phosphatases would be of high relevance for cancer biology and will improve our understanding of tyrosine phosphatases and the ability to selectively target these enzymes for therapy of human diseases.

Karen M Doody1, Nunzio Bottini1

1 Figure 1 PTPN18 regulates the tyrosine phosphorylation and ubiquitination bar- Division of Cellular Biology, La Jolla Insti- codes of HER2 at specific residues through its catalytic (PTP) and PEST domains. tute for Allergy and Immunology, La Jolla, CA 92037, USA Correspondence: Nunzio Bottini Tel: +1-858-752-6815 E-mail: [email protected] that another PEST-containing tyrosine on phosphorylation- and ubiquitination- phosphatase, PTPN22, promotes the based barcoding of the oncogene HER2 References ubiquitination of TRAF3 in innate by the tyrosine phosphatase, PTPN18 immune receptor signaling through a (Figure 1). A few questions about 1 Gschwind A, Fischer OM, Ullrich A, et al. catalytic activity-independent mecha- the molecular biology of this process Nat Rev Cancer 2004; 4:361-370. 2 Wang HM, Xu YF, Ning SL, et al. Cell Res nism [10]. The Wang et al. study goes remain open and warrant further in- 2014; 24:1067-1090. one step forward by showing that vestigation. For example, it remains to 3 Klapper LN, Waterman H, Sela M, et al. Can- PTPN18 can modify phosphorylation be determined what is the binding site cer Res 2000; 60:3384-3388. and ubiquitination of the same substrate. on HER2 that interacts with the PEST 4 Lucs AV, Muller WJ, Muthuswamy SK. On- cogene 2010; 29:174-187. The phosphorylation-dependent and the domain of PTPN18 and the composi- 5 Marone R, Hess D, Dankort D, et al. Nat Cell ubiquitination-dependent components tion of the HER2 complex that enables Biol 2004; 6:515-522. of HER2 barcoding are respectively ubiquitination and proteasomal target- 6 Dankort D, Jeyabalan N, Jones N, et al. J Biol regulated by the catalytic domain and ing of HER2. Further investigations are Chem 2001; 276:38921-38928. 7 Zhang ZY. Annu Rev Pharmacol Toxicol the PEST domain of PTPN18. This also needed to clarify whether dephos- 2002; 42:209-234. suggests that one additional feature phorylation or other post-translational 8 He R, Zeng LF, He Y, et al. FEBS J 2013; conferred to tyrosine phosphatases by modifications of the PEST domain of 280:731-750. their characteristic multi-domain or- PTPN18 can alter its function, result- 9 An H, Zhao W, Hou J, et al. Immunity 2006; 25:919-928. ganization [11] is the ability to modify ing in different barcoding of HER2 10 Wang Y, Shaked I, Stanford SM, et al. Immu- the same substrate using two different and increased HER2 half-life promoted nity 2013; 39:111-122. mechanisms. by the catalytic domain of PTPN18. 11 Alonso A, Sasin J, Bottini N, et al. Cell 2004; In summary, Wang et al. [2] present Further investigation of barcoding of 11:699-711. a study of significant scientific breadth HER2 and other signaling molecules

Cell Research | Vol 24 No 9 | September 2014