Deubiquitinating enzymes (DUBs) remove ubiquitin from conjugated substrates to regulate various cellular processes. The Zn(2+)-dependent DUBs AMSH and AMSH-LP regulate receptor trafficking by specifically cleaving Lys 63-linked polyubiquitin chains from internalized receptors. Here we report the crystal structures of the human AMSH-LP DUB domain alone and in complex with a Lys 63-linked di-ubiquitin at 1.2 A and 1.6 A resolutions, respectively. The AMSH-LP DUB domain consists of a Zn(2+)-coordinating catalytic core and two characteristic insertions, Ins-1 and Ins-2. The distal ubiquitin interacts with Ins-1 and the core, whereas the proximal ubiquitin interacts with Ins-2 and the core. The core and Ins-1 form a catalytic groove that accommodates the Lys 63 side chain of the proximal ubiquitin and the isopeptide-linked carboxy-terminal tail of the distal ubiquitin. This is the first reported structure of a DUB in complex with an isopeptide-linked ubiquitin chain, which reveals the mechanism for Lys 63-linkage-specific deubiquitination by AMSH family members.

Abstract. We previously demonstrated (Ookata et al., 1992(Ookata et al., , 1993 that the p34~¢2/cyclin B complex associates with microtubules in the mitotic spindle and premeiotic aster in starfish oocytes, and that microtubule-associated proteins (MAPs) might be responsible for this interaction. In this study, we have investigated the mechanism by which p34 ~dc2 kinase associates with the microtubule cytoskeleton in primate tissue culture cells whose major MAP is known to be MAP4. Double staining of primate cells with anticyclin B and anti-MAP4 antibodies demonstrated these two antigens were colocalized on microtubules and copartitioned following two treatments that altered MAP4 distribution. Detergent extraction before fixation removed cyclin B as well as MAP4 from the microtubules. Depolymerization of some of the cellular microtubules with nocodazole preferentially retained the microtubule localization of both cyclin B and MAP4. The association of p34~d~Vcyclin B kinase with microtubules was also shown biochemically to be mediated by MAP4. Cosedimentation of purified p34cdc2/cyclin B with purified microtubule proteins containing MAP4, but not with MAP-free microtubules, as well as binding of MAP4 to GST-cyclin B fusion proteins, demonstrated an interaction between cyclin B and MAP4. Using recombinant MAP4 fragments, we demonstrated that the Pro-rich C-terminal region of MAP4 is sufficient to mediate the cyclin B-MAP4 interaction. Since p34~cVcyclin B physically associated with MAP4, we examined the ability of the kinase complex to phosphorylate MAP4. Incubation of a ternary complex of p34 ~c2, cyclin B, and the COOHterminal domain of MAP4, Phu, with ATP resulted in intracomplex phosphorylation of PA4. Finally, we tested the effects of MAP4 phosphorylation on microtubule dynamics. Phosphorylation of MAP4 by p34 ~d~2 kinase did not prevent its binding to microtubules, but abolished its microtubule stabilizing activity. Thus, the cyclin B/MAP4 interaction we have described may be important in targeting the mitotic kinase to appropriate cytoskeletal substrates, for the regulation of spindle assembly and dynamics.p ROGaESSION through M-phase of the cell cycle is controlled by M-phase promoting factor (MPF) i, which consists of a complex of p34 cd~2 and cyclin B (for

M phase promoting factor (MPF) is a major element controlling entry into the M phase of the eukaryotic cell cycle. MPF is composed of two subunits, p34cdc2 and cyclin B. Using indirect immunofluorescence staining with specific antibody against starfish cyclin B, we monitored the dynamics of the subcellular distribution of MPF during meiosis reinitiation in starfish oocytes. We found that all of the cyclin B is already associated with p34cdc2 in immature oocytes arrested at the G2/M border and that this inactive complex is present exclusively in the cytoplasm. After its activation, part of the p34cdc2‐cyclin B complex moves into the germinal vesicle before nuclear envelope breakdown, independently of either microtubules or actin filaments. Thereafter, some part of the complex accumulates in the nucleolus and condensed chromosomes. Another portion of the complex accumulates on meiotic asters and spindles, while the rest is still present throughout the cytoplasm. As these patterns of localization are detected in the detergent‐extracted oocytes, we propose at least four distinct subcellular states of the p34cdc2‐cyclin B complex: freely soluble, microtubule‐associated, detergent‐resistant cytoskeleton‐associated and chromosome‐associated. Thus, in addition to the intramolecular modification of p34cdc2‐cyclin B complex, its intracellular relocation plays a key role in promoting the M phase.

We reported previously that cdc2 kinase decreased the microtubule-stabilizing ability of a major HeLa cell microtubule-associated protein, MAP4, by phosphorylation in vitro [Ookata, K., et al. (1995) J. Cell Biol. 128, 849-862]. An important question raised by this study is whether MAP4 is indeed phosphorylated by cdc2 kinase at mitosis in vivo. We present here evidence that cdc2 kinase is the major M-phase MAP4 kinase, and, further, we identify two phosphorylation sites within the proline-rich domain of MAP4. Metabolic 32P labeling showed the increased phosphorylation of MAP4 at mitosis. A specific inhibitor of cdc2 kinase, butyrolactone I, inhibited phosphorylation of MAP4 both in mitotic HeLa cells and in the mitotic HeLa cell extract. The phosphopeptide map analysis revealed the high similarity of in vivo labeled mitotic MAP4 to that phosphorylated by cdc2 kinase in vitro. Ser-696 and Ser-787, both of which lie within SPXK consensus sequences for cdc2 kinase, were identified as phosphorylation sites in the proline-rich region of MAP4 in vivo and in vitro. Immunoblotting with antibodies that recognize the phosphorylation state of Ser-696 or Ser-787 showed that Ser-787 in the SPSK sequence was specifically phosphorylated at mitosis while Ser-696 in the SPEK sequence was phosphorylated both at mitosis and in interphase. These results suggest that cdc2 kinase directly regulates microtubule dynamics at mitosis through phosphorylation of MAP4 at a number of sites, including Ser-787.

Mitochondrion-rich cells (MRCs), or ionocytes, play a central role in aquatic species, maintaining body fluid ionic homeostasis by actively taking up or excreting ions. Since their first description in 1932 in eel gills, extensive morphological and physiological analyses have yielded important insights into ionocyte structure and function, but understanding the developmental pathway specifying these cells remains an ongoing challenge. We previously succeeded in identifying a key transcription factor, Foxi3a, in zebrafish larvae by database mining. In the present study, we analyzed a zebrafish mutant, quadro (quo), deficient in foxi1 gene expression and found that foxi1 is essential for development of an MRC subpopulation rich in vacuolar-type H+-ATPase (vH-MRC). foxi1 acts upstream of Delta-Notch signaling that determines sporadic distribution of vH-MRC and regulates foxi3a expression. Through gain- and loss-of-function assays and cell transplantation experiments, we further clarified that (1) the expression level of foxi3a is maintained by a positive feedback loop between foxi3a and its downstream gene gcm2 and (2) Foxi3a functions cell-autonomously in the specification of vH-MRC. These observations provide a better understanding of the differentiation and distribution of the vH-MRC subtype.

A novel inwardly rectifying K+ channel, Kir7.1, with unique pore properties, was cloned recently. Working in the field of osmoregulation, we have also identified the same human and rat channel and found that the channel is unique not only in its pore sequence but also in its dense localization in the follicular cells of the thyroid gland. Northern blot analysis revealed that the channel message was abundantly expressed in the thyroid gland and small intestine, and moderately in the kidney, stomach, spinal cord and brain. Immunohistochemistry of the rat thyroid, intestine and choroid plexus demonstrated the expression of the channel protein in the follicular cells and epithelial cells, suggesting a role in the regulation of the ion-transporting functions of these specialized cells. The unique pore properties of Kir7.1 make it a strong candidate for the hypothetical low-conductance K+ channel that is functionally coupled with Na+,K(+)-ATPase by recycling K+. We therefore further examined the co-localization of Kir7.1 and Na+,K(+)-ATPase and found that both are localized in the basolateral membrane of the thyroid follicular cell; in the choroid plexus, which is known to be unique in having Na+,K(+)-ATPase in the apical side of the epithelial cells, Kir7.1 was found in the apical membrane, implying a close functional coupling between the channel and Na+,K(+)-ATPase.

A novel inwardly rectifying K+ channel, Kir7.1, with unique pore properties, was cloned recently. Working in the field of osmoregulation, we have also identified the same human and rat channel and found that the channel is unique not only in its pore sequence but also in its dense localization in the follicular cells of the thyroid gland. Northern blot analysis revealed that the channel message was abundantly expressed in the thyroid gland and small intestine, and moderately in the kidney, stomach, spinal cord and brain. Immunohistochemistry of the rat thyroid, intestine and choroid plexus demonstrated the expression of the channel protein in the follicular cells and epithelial cells, suggesting a role in the regulation of the ion-transporting functions of these specialized cells. The unique pore properties of Kir7.1 make it a strong candidate for the hypothetical low-conductance K+ channel that is functionally coupled with Na+,K(+)-ATPase by recycling K+. We therefore further examined the co-localization of Kir7.1 and Na+,K(+)-ATPase and found that both are localized in the basolateral membrane of the thyroid follicular cell; in the choroid plexus, which is known to be unique in having Na+,K(+)-ATPase in the apical side of the epithelial cells, Kir7.1 was found in the apical membrane, implying a close functional coupling between the channel and Na+,K(+)-ATPase.

MBD3, a component of the histone deacetylase NuRD complex, contains the methyl-CpG-binding domain (MBD), yet does not possess appreciable mCpG-specific binding activity. The functional significance of MBD3 in the NuRD complex remains enigmatic, partly because of the limited availability of biochemical approaches, such as immunoprecipitation, to analyze MBD3. In this study, we stably expressed the FLAG-tagged version of MBD3 in HeLa cells. We found that MBD3-FLAG was incorporated into the NuRD complex, and the MBD3-FLAG-containing NuRD complex was efficiently immunoprecipitated by anti-FLAG antibodies. By exploiting this system, we found that MBD3 is phosphorylated in vivo in the late G 2 and early M phases. Moreover, we found that Aurora-A, a serine/threonine kinase active specifically in the late G 2 and early M phases, phosphorylates MBD3 in vitro, physically associates with MBD3 in vivo, and co-localizes with MBD3 at the centrosomes in the early M phase. Interestingly, HDAC1 is distributed at the centrosomes in a manner similar to MBD3. These results suggest the highly dynamic nature of the temporal and spatial distributions, as well as the biochemical modification, of the NuRD complex in M phase, probably through an interaction with kinases, including Aurora-A. These observations will contribute significantly to the elucidation of the yet-uncharacterized cell cyclecontrolled functions of the NuRD complex.Core histones are subjected to a variety of post-translational modifications, including acetylation and methylation (1). These modifications affect the chromatin structure and/or the accessibility of transcriptional regulatory factors to chromatin and thereby influence gene expression. Acetylation has been the most extensively investigated among them. Generally, hyperacetylated histones are correlated with the transcriptionally active state, whereas hypoacetylated histones are correlated with the silent state (reviewed in Ref. 2). The relative level of histone acetylation is determined by the equilibrium between two opposite enzymatic activities, histone acetyltransferases (HATs) 1 and histone deacetylases (HDACs). Each of the HAT and HDAC groups comprises a protein family consisting of many member proteins. These proteins do not function by themselves. Instead, they form large multiprotein complexes together with other components, such as DNA/chromatin-binding proteins and co-activator/co-repressor, as well as other rather ill-defined proteins. Among the members of the HDAC family, HDAC1, HDAC2, and HDAC3 comprise one subgroup (Class I HDACs) because of their homology to yeast HDAC Rpd3. HDAC1 and HDAC2 are present as two major complexes, namely, NuRD and SIN3 (reviewed in Ref. 3). These two HDAC complexes share some components, including HDAC1 and HDAC2, and contain specific components. NuRD is unique in that it possesses another catalytic activity, the ATP-dependent chromatin-remodeling activity performed by Mi2.The acetylation of histones is not a stationary process. Instead, it is known that the genera...