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¹ Graduate School of Biostudies, Kyoto University, Yoshidakonoe-cho, Sakyo-ku, Kyoto 606-8501, Japan
² The G0 Cell Unit, Okinawa Institute of Science and Technology Promotion Corporation, Suzaki 12-22, Uruma, Okinawa 904-2234, Japan

	Abstract

Top Abstract Introduction Members of the PIKK... Schizosaccharomyces pombe PIKKs Physical interaction between... Networking by Tel2 Possible roles of Tel2... References

 
A recent paper (Hayashi et al. 2007) in this issue of Genes to Cells shows that the fission yeast Schizosaccharomyces pombe Tel2, a homologue of mammalian/worm CLK2/Clk-2/Rad-5, physically interacts with all the phosphoinositide 3-kinase-related kinases (PIKKs) that include Rad3/Tel1 (ATR/ATM homologues), Tor1/Tor2 (TOR kinases) and Tra1/Tra2 (TRRAP homologues), raising the possibility that Tel2 family proteins link various PIKK-related cellular processes by interacting with PIKK family proteins. In this minireview, implications and impact of the findings, and a possibility that PIKKs are functionally related through Tel2, are discussed.

	Introduction

Top Abstract Introduction Members of the PIKK... Schizosaccharomyces pombe PIKKs Physical interaction between... Networking by Tel2 Possible roles of Tel2... References

 
Mass spectroscopic identification of fission yeast Schizosaccharomyces pombe Tel2-binding proteins showed that Tel2 physically interacts with all the six phosphoinositide 3-kinase related kinases (PIKKs) in this organism (Hayashi et al. 2007). In this minireview, we will outline the background and discuss the impact of this finding.

	Members of the PIKK superfamily

Top Abstract Introduction Members of the PIKK... Schizosaccharomyces pombe PIKKs Physical interaction between... Networking by Tel2 Possible roles of Tel2... References

 
We describe below the basics of PIKKs. PIKKs play important roles in a variety of eukaryotic cellular functions. Members of the family include ATM, ATR that are essential in various aspects of DNA damage response, TOR/FRAP (target of rapamycin or FKBP-rapamycin-associated protein) that plays a central role in cell growth and response to nutritional environment (Abraham 2001; Wullschleger et al. 2006). Transformation/transcription domain-associated protein (TRRAP) is also structurally related to the PIKK family. TRRAP proteins (Tra1 in budding yeast) are common components of many histone acetyltransferase (HAT) complexes, and mediate a variety of cellular processes by recruiting HAT complexes to chromatin. Budding yeast Tra1 is a component of the SAGA and NuA4 histone acetyltransferase complexes that regulate transcription. Human TRRAP is a co-factor for c-Myc-mediated oncogenic transformation (Murr et al. 2007).

PIKKs contain three common domains: the FAT (FRAP, ATM and TRRAP), kinase catalytic and FATC (FRAP, ATM and TRRAP, C-terminus) (Bosotti et al. 2000) as depicted in Fig. 1A. All of the S. pombe PIKKs that have these three domains are coprecipitated with Tel2. ATM, ATR and TOR are shown to possess protein kinase activities that phosphorylate serine or threonine residues, whereas the kinase domain of TRRAP lacks the conserved amino acids required for ATP binding, and therefore TRRAP does not possess kinase activity. It was suggested that ATM is held inactive as a dimer or multimer with its kinase domain bound to a region that is overlapped with the FAT domain (Bakkenist & Kastan 2003). However, the functions of the FAT and FATC domains and their regulation remain largely unknown.

View larger version (31K): [in this window] [in a new window]   Figure 1  Tel2 associates with all the PIKKs in fission yeast. (A) Structural features of the PIKK superfamily proteins. Three common domains, the FAT, kinase catalytic and FATC, exist at the C-terminal region. (B) Tel2-interacting proteins. Tel2 interacts with PIKKs (Rad3, Tel1, Tor1, Tor2, Tra1 and Tra2) as well as with Tti1 and Tti2. Tel2 and Tti1 may form a heterodimer. Rad3 and Tel1 are involved in DNA damage and replication checkpoints, whereas Tor1 and Tor2 are required for proper cell growth and nutrient uptake. The putative function of Tra1 and Tra2 in fission yeast is regulation of transcription upon various stimuli. (C) Models for the roles of Tel2 in the PIKK signaling pathways. Upper: Tel2, as a mediator, conveys a signal from one PIKK to the other. Lower: Tel2 is a principal receiver of various signals, and influences PIKKs.

	Schizosaccharomyces pombe PIKKs

Top Abstract Introduction Members of the PIKK... Schizosaccharomyces pombe PIKKs Physical interaction between... Networking by Tel2 Possible roles of Tel2... References

	Physical interaction between Tel2 and PIKKs

Top Abstract Introduction Members of the PIKK... Schizosaccharomyces pombe PIKKs Physical interaction between... Networking by Tel2 Possible roles of Tel2... References

 
Schizosaccharomyces pombe Tel2 (spTel2) is a member of the highly conserved Tel2/Clk-2/Rad-5 family. Members of this family are multifunctional and implicated in various cellular activities (Rothman 2002). For instance, the clk-2/rad-5 gene in the nematode Caenorhabditis elegans controls genotoxic stress-induced checkpoint, DNA repair and normal ageing/biological clock. The budding yeast Saccharomyces cerevisiae Tel2 controls telomere length and telomere position effect (Runge & Zakian 1996), raising the possibility that these variously distinct biological processes are linked by the Tel2/Clk-2/Rad-5 family proteins, but ways of the link are unknown.

SpTel2 is required for cell viability and replication checkpoint. Depletion of spTel2 impairs phosphorylation of checkpoint-related Mrc1 (a Claspin homologue), which is presumed to be under the control of Rad3/ATR and Tel1/ATM (Zhao et al. 2003; Shikata et al. 2007). Similarly, the nematode Clk-2/Rad-5 functions downstream of ATL-1 (an ATR homologue) in the replication checkpoint pathway (Garcia-Muse & Boulton 2005), while human CLK2 (HCLK2) facilitates replication checkpoint, replication fork stabilization and DNA repair through interacting with ATR (Collis et al. 2007).

Mass spectroscopic analyses of the fission yeast Tor1 and Tor2 immunoprecipitates identified Tel2 and Tti1 as well as the subunits of TORC1 and TORC2, but not the other PIKK superfamily proteins. Secondly, mass spectroscopic analysis of the spTel2-immunoprecipitates identified Rad3, Tel1, Tor1, Tor2, Tra1, Tra2, Wat1/Pop3/Lst8, Tti1 and Tel two-interacting protein 2 (Tti2), as shown in Fig. 1B (Hayashi et al. 2007). Tti1/SPCC622.13c and Tti2/SPBC1604.17c are functionally unknown. Note that Tti1, Tti2 and Wat1/Pop3/Lst8 have no PIKK motif. Because HCLK2 also interacts with ATR in human cells, the above findings suggest the possibility that the interactions between the Tel2/Clk-2/Rad-5 family proteins and PIKKs are conserved among species.

	Networking by Tel2

Top Abstract Introduction Members of the PIKK... Schizosaccharomyces pombe PIKKs Physical interaction between... Networking by Tel2 Possible roles of Tel2... References

 
Followings may be relevant to consider networking by Tel2.

Tel2 immunoprecipitates contain all the S. pombe PIKK superfamily catalytic subunits in addition to Tti1, Tti2 and Wat1/Pop3/Lst8. Then, how does Tel2 interact with PIKKs? We speculate that Tel2 (and/or Tti1) interacts with the conserved domains of PIKKs, the FAT, kinase catalytic and/or FATC domains. This hypothesis can be experimentally tested.

Tel2 and Tti1 may form a heterodimer in vivo, as the level of Tti1 in the Tel2 immunoprecipitates is comparable to that of Tel2. Because they are conserved proteins, the Tel2–Tti1 complex may also exist in other organisms. The levels of Tti2 and Wat1/Pop3/Lst8 in the Tel2 immunoprecipitates were much lower than that of Tti1, and comparable to those of Tor1 and Tor2. Tti2 is also evolutionarily conserved, and a human homologue is present (Hayashi et al. 2007).

Tel2 preferentially associates with the catalytic subunits of PIKKs and may not be in the large holocomplex such as TORC1/2. Alternatively, the Tel2–Tti1 dimer is the inherent regulatory component of PIKKs catalytic subunits, and their complexes may have different functions from those of the holoenzymes.

	Possible roles of Tel2 in the PIKKs signal transduction

Top Abstract Introduction Members of the PIKK... Schizosaccharomyces pombe PIKKs Physical interaction between... Networking by Tel2 Possible roles of Tel2... References

 
We discuss below how Tel2 affects signal transductions of PIKKs. Cross-talks may exist between different PIKKs that are linked by Tel2 (and Tti1). An alteration in one PIKK may influence other PIKKs through the presumed mediator role of Tel2. Alternatively, environmental or internal stress may first affect Tel2, followed by the alterations of different PIKKs (Fig. 1C). Nutrient response versus DNA damage response may be intricately related via Tel2 (and possibly Tti1). Indeed Shen et al. reported that TORC1 is required for the S-phase progression and cell survival in response to genotoxic stress in budding yeast (Shen et al. 2007). In fission yeast, both Tor1 and Tor2 are involved in the survival following replication fork stalling (Uritani et al. 2006). Various stresses such as DNA damage, cell cycle perturbation, nutritional starvation and senescence/ageing factors may be received and coordinately responded by the signaling of PIKKs through the presumed liaison role of Tel2 (and Tti1).

Further study is definitely needed to understand the mechanistic role of Tel2. ATM is known to dramatically change its conformation, dimer or multimer to monomer, following DNA damage (Bakkenist & Kastan 2003). Tel2 may be implicated in such conformational change of PIKKs in response to various stimuli. Another possibility is that Tel2 is a common stabilizer of PIKKs, or Tel2 may serve as a scaffold protein that mediates signal transduction from PIKKs to their target proteins, like those for MAP kinase cascades (Dard & Peter 2006).

In short, Tel2 (and Tti1) is an attractive protein that participates in and may link the cellular processes operated by PIKKs. The diverse phenotypes of the tel2/clk-2/rad-5 mutants in yeasts and nematode can now be explained by the interactions of Tel2 with PIKKs. Further study will reveal the molecular functions of the Tel2/Clk-2/Rad-5 family proteins, leading to a discovery of unexpected coordination of PIKK signalings that may have been conserved during the course of evolution.

	Acknowledgements

 
J. K. was supported by a Grant-in-Aid for Scientific Research on Priority Areas from the Ministry of Education, Culture, Sports, Science and Technology of Japan.

^* Correspondence: E-mail: jkanoh{at}lif.kyoto-u.ac.jp

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