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Members of the NDR (nuclear Dbf2-related) protein-kinase family are essential components of pathways that control important cellular processes, such as morphological changes, mitotic exit, cytokinesis, cell proliferation and apoptosis. Recent progress has shed light on the mechanisms that underlie the regulation and function of the NDR family members. Combined data from yeast, worms, flies, mice and human cells now highlight the conserved and important roles of the different NDR kinases in distinct cellular processes.

Von Hippel-Lindau (VHL) tumour suppressor gene inactivation is linked to the development of haemangioblastomas in the central nervous system and retina, often in association with other tumours, such as clear-cell carcinomas of the kidney and phaeochromocytomas. Here we show that the VHL protein (pVHL) is a microtubule-associated protein that can protect microtubules from depolymerization in vivo. Both the microtubule binding and stabilization functions of pVHL depend on amino acids 95-123 of pVHL, a mutational 'hot-spot' in VHL disease. From analysis of naturally occurring pVHL mutants, it seems that only point mutations such as pVHL(Y98H) and pVHL(Y112H) (that predispose to haemangioblastoma and phaeochromocytoma, but not to renal cell carcinoma) disrupt pVHL's microtubule-stabilizing function. Our data identify a role for pVHL in the regulation of microtubule dynamics and potentially provide a link between this function of pVHL and the pathogenesis of haemangioblastoma and phaeochromocytoma in the context of VHL disease.

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Human NDR1 and 2 (NDR1/2) are serine-threonine protein kinases in a subgroup of the AGC kinase family. The mechanisms of physiological NDR1/2 activation and their function remain largely unknown. Here we report that Fas and TNF-alpha receptor stimulation activates human NDR1/2 by promoting phosphorylation at the hydrophobic motif (Thr444/442). Moreover, NDR1/2 are essential for Fas receptor-induced apoptosis as shown by the fact that NDR knockdown significantly reduced cell death whereas overexpression of the NDR1 kinase further potentiated apoptosis. Activation of NDR1/2 by death receptor stimulation is mediated by the tumor suppressor RASSF1A. Furthermore, RASSF1A-induced apoptosis largely depends on the presence of NDR1/2. Fas receptor stimulation promoted direct phosphorylation and activation of NDR1/2 by the mammalian STE20-like kinase 1 (MST1), a downstream effector of RASSF1A. Concurrently, the NDR1/2 coactivator MOB1 induced MST1-NDR-MOB1 complex formation, which is crucial for MST1-induced NDR1/2 phosphorylation upon induction of apoptosis. Our findings identify NDR1/2 as novel proapoptotic kinases and key members of the RASSF1A/MST1 signaling cascade.

Human nuclear Dbf2-related kinases (NDRs) are up-regulated in certain cancer types, yet their precise function(s) and regulatory mechanism(s) still remain to be defined. Here, we show that active (phosphorylated on Thr444) and inactive human NDRs are both mainly cytoplasmic. Moreover, NDR kinases colocalize at the plasma membrane with human MOBs (hMOBs), which are recently described coactivators of human NDR in vitro. Strikingly, membrane targeting of NDR results in a constitutively active kinase due to phosphorylation on Ser281 and Thr444 that is further activated upon coexpression of hMOBs. Membrane-targeted hMOBs also robustly promoted activation of NDR. We further demonstrate that the in vivo activation of human NDR by membrane-bound hMOBs is dependent on their interaction and occurs solely at the membrane. By using a chimeric molecule of hMOB, which allows inducible membrane translocation, we found that NDR phosphorylation and activation at the membrane occur a few minutes after association of hMOB with membranous structures. We provide insight into a potential in vivo mechanism of NDR activation through rapid recruitment to the plasma membrane mediated by hMOBs.The human genome encodes two highly related serine-threonine kinases, nuclear Dbf2-related kinase 1 (NDR1) and NDR2, which belong to a subfamily of kinases with roles in cell division and cell morphology (22). The two kinases represent a subclass of the AGC family of protein kinases and are partially related to protein kinase B (PKB) and protein kinase C (PKC). Human NDRs, as well as their yeast counterpart Dbf2, can be dramatically activated through inhibition of protein phosphatase 2A (PP2A) by okadaic acid (OA), demonstrating that NDR kinases require phosphorylation for activation (13,16). Indeed, NDR1 has been shown to become autophosphorylated on Ser281 in a Ca 2ϩ -dependent manner (23), while Thr444 is phosphorylated by a hydrophobic motif kinase (M. R. Stegert et al., submitted for publication). Phosphorylation on both residues is essential for full activation of NDR1 in vitro and in vivo (16,23). Recently, the phosphorylation of NDR2 on Ser282 and Thr442 was also shown to be required for full activation of enzymatic activity (20).The founding member of the Mps1 one binder (MOB) family is the yeast protein MOB1p (12), a molecule functioning as a coactivator of yeast Dbf2p and Dbf20p (10,11,13). As previously published, human MOB1A (hMOB1A) binds NDR1 and NDR2 (NDRs) at their N termini, leading to kinase activation in vitro (3). In addition, hMOB1B and hMOB2 also stimulated NDR activity in vitro (4). However, while the MOB regulation of the NDR kinases seems to be conserved from yeast to humans, the function(s) of mammalian NDRs is unknown. Although the basic mechanism of activation of human NDR protein kinase in vitro has been described in detail (3,16,20,23), the regulation of NDR in a cellular context still remains to be elucidated. Such information should bring insight into its cellular functions and, in particular, into its possible role ...

The family of Mps One binder (MOB) co-activator proteins is highly conserved from yeast to man. At least two different MOB proteins have been identified in every eukaryote analysed to date. Initially, yeast genetics revealed essential roles for Mob1p and Mob2p in the regulation of mitotic exit and cell morphogenesis. Studies in flies then showed that dMOB1/MATS is a core component of Hippo signalling. Loss of dMOB1 resulted in increased cell proliferation and decreased cell death, suggesting that MOB1 acts as tumour suppressor protein. Recent work focused primarily on mammalian cells has shown how hMOB1 can regulate NDR/LATS kinases, a function that can to be counteracted by hMOB2. Here we summarise and discuss our current knowledge of this emerging protein family, with emphasis on subcellular localisation, protein-protein interactions and biological functions in apoptosis, mitosis, morphogenesis, cell proliferation and centrosome duplication.

Our observations propose a novel pathway in control of human centriole duplication after recruitment of HsSAS-6 to centrioles.