the family of mps one binder proteins ( mobs ) is highly conserved in eukaryotes .
mobs represent signal transducers in essential intracellular signaling pathways through their regulatory interactions with serine / threonine protein kinases of the ndr / lats family [ 13 ] . in budding and fission yeast , mob1p and mob2p are crucial for mitotic exit and cell morphogenesis as regulators of the yeast ndr / lats
kinases dbf2p , cbk1p , sid2p and orb6p , respectively [ 46 ] . in drosophila ,
three different mob proteins are expressed by independent genes , with dmob1 ( aka mats ) functioning as a core component of the hippo tumor suppressor pathway as regulator of the fly lats kinase warts [ 79 ] .
drosophila mob2 ( dmob2 ) contributes to neuromuscular junction and photoreceptor morphology   and the biological function(s ) of dmob3 is currently unknown , although all three dmobs can genetically interact with the fly ndr kinase tricornered .
mammalian genomes contain at least six different mob genes termed mob1a , mob1b , mob2 , mob3a , mob3b and mob3c .
mob1a / b likewise to dmob1 functions as a regulator of lats kinases in mammalian hippo signaling , although current evidence proposes that the interaction of mob1 with ndr kinases is likely to also play a role in hippo signaling .
in contrast to mob1 , mob2 interacts specifically with ndr , but not with lats kinases in mammalian cells [ 1315 ] .
mob3a / b / c neither form a complex with ndr nor lats kinases , but instead associate with the pro - apoptotic kinase mst1 ( aka stk4 ) .
taken together , throughout the eukaryotic kingdom mobs can play diverse roles as regulators of members of the ndr / lats kinase family and apparently also other protein kinases in specific settings . in this article
we will focus on discussing recent discoveries regarding roles of endogenous mob2 in cell cycle progression and the dna damage response ( ddr ) in the context ndr kinase signaling .
up to recently , mammalian ndr kinases were the only reported binding partners of mob2 .
more precisely , biochemical experiments showed that mob2 competes with mob1 for ndr binding , with the mob1/ndr complex corresponding to increased ndr kinase activity and the mob2/ndr complex being associated with diminished ndr activity . in other words ,
mob2 binding to ndr can block the activation of ndr kinases . however , the biological significance of mob2/ndr complex formation is currently unknown .
actually , no clearly defined physiologically relevant functions of endogenous mob2 were known until recently .
intriguingly , a genome wide screen for novel putative ddr factors identified mob2 ( also termed hcca2 and hmob3 ) as one of many potential candidates . considering that the ddr is essential to maintain genome stability and functions as a barrier for ageing and tumorigenesis , we investigated whether mob2 is indeed a ddr protein .
intriguingly , we initially found that mob2 knockdown , but not mob2 overexpression , caused a cell proliferation defect associated with a g1/s cell cycle arrest in untransformed human cells . by profiling an array of cell cycle markers we discovered that mob2-depleted cells displayed a significant activation of the p53 and p21/cip1 cell cycle regulators , which was functionally relevant , since co - knockdown of p53 or p21 together with mob2 did not result in the activation of the g1/s cell cycle checkpoint , consequently restoring cell proliferation
once we had established that mob2 knockdown causes the activation of a p53/p21-dependent g1/s cell cycle checkpoint , we wondered how this activation occurred . considering that endogenous mob2 is potentially linked to the ddr   and that activation of the p53/p21 pathway can occur upon activation of ddr signaling [ 2022 ] , we studied the levels of ddr signaling and endogenous dna damage in mob2-depleted cells .
these investigations revealed that mob2 knockdown causes the accumulation of dna damage , and consequently activation of the ddr kinases atm and chk2 , in the absence of exogenously induced dna damage .
next , we aimed to consolidate these findings by studying the response of mob2 knockdown cells to exogenously induced dna damage .
this showed that endogenous mob2 is needed to promote cell survival and g1/s cell cycle arrest upon exposure to dna damaging agents such as ionizing radiation ( ir ) or the topoisomerase ii poison doxorubicin .
moreover , we discovered that mob2 is required to support ir - induced ddr signaling through the ddr kinase atm . collectively , these observations uncovered endogenous mob2 as a novel ddr factor that plays a role in ddr signaling , cell survival and cell cycle checkpoints upon exposure to dna damage .
however , we still had not understood how mob2 may function as ddr protein on a molecular level . in this
regard , using a yeast two - hybrid screen we had identified rad50 as a novel binding partner of mob2 , which potentially was important , since rad50 is a central component of the essential mre11-rad50-nbs1 ( mrn ) dna damage sensor complex , which in turn is crucial for the sequestering / activation of the ddr kinase atm at dna lesions [ 2325 ]
. therefore , we examined this potential interaction in more detail , revealing that mob2/rad50 complex formation can be detected using exogenous and endogenous proteins .
moreover , we found that mob2 supports the recruitment of mrn and activated atm to dna damaged chromatin , suggesting that mob2-depleted cells display a defective ddr due to impaired functionality of the mrn .
although we could map the binding sites of mob2 on rad50 to two functionally relevant domains of rad50 , we did not succeed in identifying mob2 variants carrying single point mutations that block mob2/rad50 complex formation [ gomez v and hergovich a , unpublished observation ] , which would have enabled us to investigate the functional significance of the mob2/rad50 interaction in more detail .
therefore , our study   could not conclusively establish that the interaction of mob2 with rad50 is functionally essential for the roles of mob2 in ddr signaling , cell survival and cell cycle checkpoints upon exposure to dna damage . in this regard , it is noteworthy that in the genome wide screen performed by elledge et al
.   the knockdown of mrn components did not result in ddr defects ( as judged by sensitivity to mitomycin c and a defective g2/m dna damage checkpoint ) as observed in mob2-depleted transformed human cells .
thus , the link of mob2 to the mrn does not appear to be relevant in all ddr settings , suggesting that additional mechanisms should be considered .
in general , we have only begun to appreciate the cell biological functions of endogenous mob2 in processes that are relevant to human health and disease . in particular regarding the link of mob2 with the ddr , quite a few key questions are yet to be understood .
for example , we have yet to comprehend which types of dna damage repair mechanism(s )   is dependent on normal mob2 levels . maybe the expression / localization status of mob2 has the potential to be clinically exploited for the prediction and/or prognosis of responses to dna damaging agents ( i.e. radiotherapy , dna damaging chemotherapeutics , targeted ddr inhibition , and others ) .
furthermore , we have yet to obtain a clear mechanistic understanding of how mob2 can function as a ddr protein and how mob2 is regulated in a context - dependent manner .
nevertheless , our previous   biochemical characterization of mob2 in complex with the ndr1/2 kinases may be of help to lead some of the way .
however , based on our own experiments we already speculated that mob2 apparently functions as cell cycle / ddr regulator independently of ndr1/2 kinase signaling .
more specifically , we observed that knockdown of ndr1 ( aka stk38 ) or ndr2 ( stk38l ) in untransformed human cells did not trigger a p53/p21-dependent g1/s cell cycle arrest as observed in mob2-depleted cells .
overexpression of hyperactive ndr1-pif   also did not cause an obvious cell cycle / proliferation defect .
however , we believe that further investigations are still required to completely rule out that ndr1/2 signaling is not linked to cell cycle / ddr processes through mob2 , since different reports have recently linked the ndr1/2 kinases to cell cycle and ddr signaling .
in addition , compensatory mechanisms may occur upon selective ndr1 or ndr2 manipulations . in mammals ,
the ndr1/2 protein kinases have been linked to cell biological processes such as cell cycle progression , the ddr , apoptosis , stress signaling and autophagy , with important roles in embryogenesis , immunology and neurobiology .
as mentioned above , in particular the connections of ndr1/2 with the cell cycle and ddr are intriguing with respect to mob2 ( figure 1 ) .
on the one hand , ndr1/2 are linked to the regulation of g1/s cell cycle progression by controlling protein levels of c - myc and p21/cip1 [ 2932 ] .
the role of ndr1/2 in the g1/s cell cycle progression is further supported by cyclin d1   and can have a role in opposing a tgf-mediated cell cycle arrest .
thus , various tissue culture cell experiments support the notion that ndr1/2 can play diverse roles in the regulation of cell cycle progression . on the other hand ,
ndr1 possibly has a function in nucleotide excision repair , a specific type of dna damage repair .
in addition , ndr1 potentially is involved in the dna damage induced g2/m cell cycle checkpoint by phosphorylating the cdc25a phosphatase , although this phosphorylation of cdc25a is also mediated by the ddr kinase chk1 , hence warranting future investigations into this possible link of ndr1 and the dna damage induced g2/m cell cycle checkpoint . taken together , current evidence suggests that the ndr1/2 kinase pathway is linked to the regulation of certain aspects of cell cycle progression and signal transduction in response to dna damage ( figure 1 ) .
nevertheless , in spite of the involvement of ndr1/2 in the cell cycle and ddr   in a similar fashion as observed for mob2 , it is currently unknown whether the mob2 and ndr1/2 pathways are functionally connected , as suggested by our recent biochemical evidence . in this regard
, mob2 may act upstream of ndr1/2 by functioning as inhibitor of mob1-mediated ndr1/2 signaling .
however , simply based on our current lack of evidence , we should not exclude the possibility that ndr1/2 may play a role upstream of mob2 , in which case it would be very informative to understand the involvement of the ndr1/2 kinase activity , in addition to the regulation of mob2 by ndr1/2 ( or vice versa ) through direct protein - protein interactions . in this regard
, experimenters should also keep in mind that single ndr1 or ndr2 knockdown compared to co - depletion of ndr1 and ndr2 may result in different phenotypes due to possible compensatory mechanisms .
more specifically , we found that single ndr1 or ndr2 knockout mice are viable and fertile due to compensatory tissue specific up regulation of ndr2 or ndr1 , respectively . in contrast
these findings collectively suggest that ndr1 and ndr2 can compensate for each other in a context- and tissue - specific fashion . in case mob2 functions negatively upstream of ndr1/2 in ddr signaling , one would expect that mob2 knockdown or hyperactivation of ndr1/2 result in similar phenotypes , which does not seem to be the case . if ndr1/2 were to act upstream of mob2 in ddr signaling , one would predict that positive regulators of ndr1/2 , such as mob1 , may also contribute to the ddr .
interestingly , this seems to be the case , since mob1a or mob1b knockdown appears to be sufficient to cause spontaneous dna double - strand break formation in human cells , proposing that the regulation of ndr1/2 by mob1 might also play a role in the ddr .
however , whether ndr1/2 can function upstream of mob2 in cell cycle and/or ddr signaling is yet to be established experimentally .
the possible involvement of mob1 in ndr1/2-mob2 signaling is also of purely speculative nature at the moment , in particular when considering that mob1 can associate with different kinases of the hippo core cassette such as lats1/2 and mst1/2 . in this context
, we also would like to emphasis the fact that currently the molecular ( structural ) regulation of ndr1/2 kinases by mob1 vs. mob2 is incompletely understood .
possibly , mob2 is part of positive and/or negative feedback loops that serve to amplify and/or dampen cell cycle and/or ddr signaling , respectively .
certainly , these speculative points illustrate the need for more intensified experimental efforts to understand mob2 as a novel ddr protein on the structural , molecular , cellular and organismal level in the context of human biology in health and disease .
 this article is the authors opinion of the roles of the signal transducer mps one binder 2 ( mob2 ) in the control of cell cycle progression and the dna damage response ( ddr ) . 
 we recently found that endogenous mob2 is required to prevent the accumulation of endogenous dna damage in order to prevent the undesired , and possibly detrimental , activation of cell cycle checkpoints . in this regard , 
 it is noteworthy that mob2 has been linked biochemically to the regulation of the ndr1/2 ( aka stk38/stk38l ) protein kinases , which themselves have functions at different steps of the cell cycle . therefore , we are speculating in this article about the possible connections of mob2 with ndr1/2 kinases in cell cycle and ddr signaling . 
Introduction