In intestinal epithelial cells, inactivation of APC, a key regulator of the Wnt pathway, activates beta-catenin to initiate tumorigenesis. However, other alterations may be involved in intestinal tumorigenesis. Here we found that RUNX3, a gastric tumor suppressor, forms a ternary complex with beta-catenin/TCF4 and attenuates Wnt signaling activity. A significant fraction of human sporadic colorectal adenomas and Runx3(+/-) mouse intestinal adenomas showed inactivation of RUNX3 without apparent beta-catenin accumulation, indicating that RUNX3 inactivation independently induces intestinal adenomas. In human colon cancers, RUNX3 is frequently inactivated with concomitant beta-catenin accumulation, suggesting that adenomas induced by inactivation of RUNX3 may progress to malignancy. Taken together, these data demonstrate that RUNX3 functions as a tumor suppressor by attenuating Wnt signaling.

Members of the syntaxin protein family participate in the docking-fusion step of several intracellular vesicular transport events. Tlg1p has been identified as a nonessential protein required for efficient endocytosis as well as the maintenance of normal levels of trans-Golgi network proteins. In this study we independently describe Tlg1p as an essential protein required for cell viability. Depletion of Tlg1p in vivo causes a defect in the transport of the vacuolar protein carboxypeptidase Y through the early Golgi. Temperature-sensitive (ts) mutants of Tlg1p also accumulate the endoplasmic reticulum/cis-Golgi form of carboxypeptidase Y at the nonpermissive temperature (38 degrees C) and exhibit underglycosylation of secreted invertase. Overexpression of Tlg1p complements the growth defect of vti1-11 at the nonpermissive temperature, whereas incomplete complementation was observed with vti1-1, further suggesting a role for Tlg1p in the Golgi apparatus. Overexpression of Sed5p decreases the viability of tlg1 ts mutants compared with wild-type cells, suggesting that tlg1 ts mutants are more susceptible to elevated levels of Sed5p. Tlg1p is able to bind His6-tagged Sec17p (yeast alpha-SNAP) in a dose-dependent manner and enters into a SNARE complex with Vti1p, Tlg2p, and Vps45p. Morphological analyses by electron microscopy reveal that cells depleted of Tlg1p or tlg1 ts mutants incubated at the restrictive temperature accumulate 40- to 50-nm vesicles and experience fragmentation of the vacuole.

Microtubule dynamics is essential for many vital cellular processes such as morphogenesis and motility. Protein kinase CK2 is a ubiquitous protein kinase that is involved in diverse cellular functions. CK2 holoenzyme is composed of two catalytic ␣ or ␣ subunits and two regulatory ␤ subunits. We show that the ␣ subunit of CK2 binds directly to both microtubules and tubulin heterodimers. CK2 holoenzyme but neither of its individual subunits exhibited a potent effect of inducing microtubule assembly and bundling. Moreover, the polymerized microtubules were strongly stabilized by CK2 against cold-induced depolymerization. Interestingly, the kinase activity of CK2 is not required for its microtubule-assembling and stabilizing function because a kinase-inactive mutant of CK2 displayed the same microtubule-assembling activity as the wild-type protein.Knockdown of CK2␣/␣ in cultured cells by RNA interference dramatically destabilized their microtubule networks, and the destabilized microtubules were readily destructed by colchicine at a very low concentration. Further, over-expression of chicken CK2␣ or its kinaseinactive mutant in the endogenous CK2␣/␣-depleted cells fully restored the microtubule resistance to the low dose of colchicine. Taken together, CK2 is a microtubule-associated protein that confers microtubule stability in a phosphorylation-independent manner. Protein kinase CK2 (formerly known as casein kinase 2) is ubiquitously expressed and highly conserved in eukaryotic cells (1-4). It comprises two catalytic ␣ or ␣Ј subunits and two regulatory ␤ subunits to form a heterotetrameric structure in which the two ␤ subunits dimerize to link the two ␣ or ␣Ј subunits (5). As a protein serine/threonine kinase, CK2 has a very broad phosphorylation spectrum, and over 300 protein substrates of CK2 have been identified to date (6). A number of studies have indicated that CK2 is involved in a wide variety of cellular processes including cell cycle, apoptosis, transcriptional regulation, and signal transduction (1, 3, 6). CK2 is instrumental and necessary for promoting cell survival (3, 7). Disruption of genes encoding both of the catalytic subunits of CK2 is synthetic lethal in fission yeast (8,9). Similarly, it is embryonic lethal when CK2␤ is knocked down in Caenorhabditis elegans by RNA interference or in mice by gene disruption, reminiscent of an essential role of CK2␤ during embryonic development and organogenesis (10, 11). Hence, production of both the ␣ and ␤ subunits of CK2 appears to be mandatory for cell viability.A few lines of evidence have lead to implication that CK2 might be involved in the regulation of microtubule cytoskeleton reorganization (12-14). CK2 was localized to microtubule structures such as the mitotic spindle of dividing cells and was found to associate with the cold-stable fraction of microtubules from the rat brain (14, 15). More recently, the ␣ and ␣Ј subunits were shown to bind tubulin in a far Western assay (16). Further, CK2 is able to phosphorylate a number of microtubule elements, in...

Cdk5 is a unique member of the cyclin-dependent kinase (Cdk) family of small protein kinases. In association with its neuron-specific activator p35 or p39, Cdk5 displays many regulatory properties distinct from other Cdks. A growing body of evidence has suggested that Cdk5-p35 has important implications in a variety of neuronal activities occurring in the central nervous system. In brain, Cdk5-p35 appears to exist as large molecular complexes with other proteins, and protein-protein interactions appear to be a molecular principle for Cdk5-p35 to conduct its physiological functions. Over the past decade, a number of proteins have been identified to associate with Cdk5-p35. While the majority of these proteins mediate their interaction with Cdk5 through p35, implying that p35 may act not only as an activator of Cdk5 but also as an adaptor to associate Cdk5 with its regulators and physiological targets, a small group of other proteins are found to link directly with Cdk5. In addition, Cdk5 has been found to phosphorylate a diverse list of substrates, further implicating its regulatory roles in a wide range of cellular processes. In this review, we present an updated inventory of the interacting proteins of Cdk5-p35 kinase and its substrates as well as a discussion on the implicated effects of these interactions.

The complex of Cdk5 and its neuronal activator p35 is a proline-directed Ser/Thr kinase that plays an important role in various neuronal functions. Deregulation of the Cdk5 enzymatic activity was found to associate with a number of neurodegenerative diseases. To search for regulatory factors of Cdk5-p35 in the brain, we developed biochemical affinity isolation using a recombinant protein comprising the N-terminal 149 amino acids of p35. The catalytic ␣-subunit of protein kinase CK2 (formerly known as casein kinase 2) was identified by mass spectrometry from the isolation. The association of CK2 with p35 and Cdk5 was demonstrated, and the CK2-binding sites were delineated in p35. Furthermore, CK2 displayed strong inhibition toward the Cdk5 activation by p35. The Cdk5 inhibition is dissociated from the kinase function of CK2 because the kinase-dead mutant of CK2 displayed the similar Cdk5 inhibitory activity as the wild-type enzyme. Further characterization showed that CK2 blocks the complex formation of Cdk5 and p35. Together, these findings suggest that CK2 acts as an inhibitor of Cdk5 in the brain.Cdk5 was identified independently on the basis of its sequence similarity to the family of Cdks, its interaction with cyclin D, and its protein kinase activity toward a prolinedirected Ser/Thr sequence (1-3). Despite having 60% sequence identity with Cdk1 and Cdk2, a role for Cdk5 in cell cycle regulation has yet to be identified. Nevertheless, Cdk5 has been implicated in the regulation of neuronal differentiation, degeneration, and cytoskeletal dynamics (4). Monomeric Cdk5 shows no enzymatic activity because its activation is dependent on its association with the regulatory subunit p35 or p39, neither of which is a cyclin protein (5-7). Mammalian p35 and p39 are primarily expressed in post-mitotic neurons, thereby restricting Cdk5 activity predominantly to the nervous system (8). Cdk5 and p35 are essential for proper brain development. Mice deficient of Cdk5 or p35 display cell-positioning defects in the cerebral cortex (9, 10). In the brain, Cdk5-p35 phosphorylates a number of cytoskeletal proteins that are thought to play important roles in the reassembly of cytoskeletal elements, thereby mediating neurite outgrowth and neuronal migration during the development (11-16). In addition, there are several lines of evidence implicating aberrant regulation of Cdk5 in neurodegeneration and cell death (17). Indeed, p25, a truncated C-terminal fragment of p35, was found to accumulate in Alzheimer's disease brains; its associated-Cdk5 kinase activity was shown to lead to cytoskeletal disruption, morphological degeneration, and apoptosis (18 -21).Cdk5 is involved in the regulation of many vital functions in the brain and, hence, its activity needs to be tightly controlled in the cells. Upon association with its activator p35 or p39, Cdk5 is regulated through various means, and many of its regulatory properties are distinct from those of the authentic Cdk-cyclin. First, p35 is an unstable protein with a half-life of 20 -30 min ...

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