N-terminal region of ZW10 serves not only as a determinant for localization but also as a link with dynein function

Dynein at the kinetochore

The microtubule minus-end-directed motility of cytoplasmic dynein 1 (dynein), arguably the most complex and versatile cytoskeletal motor, is harnessed for diverse functions, such as long-range organelle transport in neuronal axons and spindle assembly in dividing cells. The versatility of dynein raises a number of intriguing questions, including how is dynein recruited to its diverse cargo, how is recruitment coupled to activation of the motor, how is motility regulated to meet different requirements for force production and how does dynein coordinate its activity with that of other microtubule-associated proteins (MAPs) present on the same cargo. Here, these questions will be discussed in the context of dynein at the kinetochore, the supramolecular protein structure that connects segregating chromosomes to spindle microtubules in dividing cells. As the first kinetochore-localized MAP described, dynein has intrigued cell biologists for more than three decades. The first part of this Review summarizes current knowledge about how kinetochore dynein contributes to efficient and accurate spindle assembly, and the second part describes the underlying molecular mechanisms and highlights emerging commonalities with dynein regulation at other subcellular sites.

Rs2459976 in ZW10 gene associated with congenital heart diseases in Chinese Han population

Congenital heart diseases (CHD) are a large group of prevalent and complex anatomic malformations of the heart, with the genetic basis remaining largely unknown. Since genes or factors associated with the differentiation of human embryonic stem (HES) cells would affect the development of all embryonic tissues, including cardiac progenitor cells, we postulated their potential roles in CHD. In this study, we focused on ZW10, a kinetochore protein involved in the process of proper chromosome segregation, and conducted comparative studies between CHD patients and normal controls matched in gender and age in Chinese Han populations. We identified three variations in the ZW10 gene, including rs2885987, rs2271261 and rs2459976, which all had high genetic heterozygosity. Association analysis of these genetic variations with CHD showed correlation between rs2459976 and the risk of CHD. We conclude that rs2459976 in the ZW10 gene is associated with CHD in Chinese Han populations.

Rint1 inactivation triggers genomic instability, ER stress and autophagy inhibition in the brain

Endoplasmic reticulum (ER) stress, defective autophagy and genomic instability in the central nervous system are often associated with severe developmental defects and neurodegeneration. Here, we reveal the role played by Rint1 in these different biological pathways to ensure normal development of the central nervous system and to prevent neurodegeneration. We found that inactivation of Rint1 in neuroprogenitors led to death at birth. Depletion of Rint1 caused genomic instability due to chromosome fusion in dividing cells. Furthermore, Rint1 deletion in developing brain promotes the disruption of ER and Cis/Trans Golgi homeostasis in neurons, followed by ER-stress increase. Interestingly, Rint1 deficiency was also associated with the inhibition of the autophagosome clearance. Altogether, our findings highlight the crucial roles of Rint1 in vivo in genomic stability maintenance, as well as in prevention of ER stress and autophagy.

Moonlighting functions of the NRZ (mammalian Dsl1) complex

The yeast Dsl1 complex, which comprises Dsl1, Tip20, and Sec39/Dsl3, has been shown to participate, as a vesicle-tethering complex, in retrograde trafficking from the Golgi apparatus to the endoplasmic reticulum. Its metazoan counterpart NRZ complex, which comprises NAG, RINT1, and ZW10, is also involved in Golgi-to-ER retrograde transport, but each component of the complex has diverse cellular functions including endosome-to-Golgi transport, cytokinesis, cell cycle checkpoint, autophagy, and mRNA decay. In this review, we summarize the current knowledge of the metazoan NRZ complex and discuss the "moonlighting" functions and intercorrelation of their subunits.

The Drosophila RZZ complex - roles in membrane trafficking and cytokinesis

The Zw10 protein, in the context of the conserved Rod-Zwilch-Zw10 (RZZ) complex, is a kinetochore component required for proper activity of the spindle assembly checkpoint in both Drosophila and mammals. In mammalian and yeast cells, the Zw10 homologues, together with the conserved RINT1/Tip20p and NAG/Sec39p proteins, form a second complex involved in vesicle transport between Golgi and ER. However, it is currently unknown whether Zw10 and the NAG family member Rod are also involved in Drosophila membrane trafficking. Here we show that Zw10 is enriched at both the Golgi stacks and the ER of Drosophila spermatocytes. Rod is concentrated at the Golgi but not at the ER, whereas Zwilch does not accumulate in any membrane compartment. Mutations in zw10 and RNAi against the Drosophila homologue of RINT1 (rint1) cause strong defects in Golgi morphology and reduce the number of Golgi stacks. Mutations in rod also affect Golgi morphology, whereas zwilch mutants do not exhibit gross Golgi defects. Loss of either Zw10 or Rint1 results in frequent failures of spermatocyte cytokinesis, whereas Rod or Zwilch are not required for this process. Spermatocytes lacking zw10 or rint1 function assemble regular central spindles and acto-myosin rings, but furrow ingression halts prematurely due to defective plasma membrane addition. Collectively, our results suggest that Zw10 and Rint1 cooperate in the ER-Golgi trafficking and in plasma membrane formation during spermatocyte cytokinesis. Our findings further suggest that Rod plays a Golgi-related function that is not required for spermatocyte cytokinesis.

Golgi positioning

The Golgi apparatus in mammalian cells is positioned near the centrosome-based microtubule-organizing center (Fig. 1). Secretory cargo moves inward in membrane carriers for delivery to Golgi membranes in which it is processed and packaged for transport outward to the plasma membrane. Cytoplasmic dynein motor proteins (herein termed dynein) primarily mediate inward cargo carrier movement and Golgi positioning. These motors move along microtubules toward microtubule minus-ends embedded in centrosomes. Centripetal motility is controlled by a host of regulators whose precise functions remain to be determined. Significantly, a specific Golgi receptor for dynein has not been identified. This has impaired progress toward elucidation of membrane-motor-microtubule attachment in the periphery and, after inward movement, recycling of the motor for another round. Pericentrosomal positioning of the Golgi apparatus is dynamic. It is regulated during critical cellular processes such as mitosis, differentiation, cell polarization, and cell migration. Positioning is also important as it aligns the Golgi along an axis of cell polarity. In certain cell types, this promotes secretion directed to the proximal plasma membrane domain thereby maintaining specializations critical for diverse processes including wound healing, immunological synapse formation, and axon determination.

Structural analysis of the RZZ complex reveals common ancestry with multisubunit vesicle tethering machinery

The RZZ complex recruits dynein to kinetochores. We investigated structure, topology, and interactions of the RZZ subunits (ROD, ZWILCH, and ZW10) in vitro, in vivo, and in silico. We identify neuroblastoma-amplified gene (NAG), a ZW10 binder, as a ROD homolog. ROD and NAG contain an N-terminal beta propeller followed by an alpha solenoid, which is the architecture of certain nucleoporins and vesicle coat subunits, suggesting a distant evolutionary relationship. ZW10 binding to ROD and NAG is mutually exclusive. The resulting ZW10 complexes (RZZ and NRZ) respectively contain ZWILCH and RINT1 as additional subunits. The X-ray structure of ZWILCH, the first for an RZZ subunit, reveals a novel fold distinct from RINT1's. The evolutionarily conserved NRZ likely acts as a tethering complex for retrograde trafficking of COPI vesicles from the Golgi to the endoplasmic reticulum. The RZZ, limited to metazoans, probably evolved from the NRZ, exploiting the dynein-binding capacity of ZW10 to direct dynein to kinetochores.

Cdc42 regulates microtubule-dependent Golgi positioning

The molecular mechanisms underlying cytoskeleton-dependent Golgi positioning are poorly understood. In mammalian cells, the Golgi apparatus is localized near the juxtanuclear centrosome via dynein-mediated motility along microtubules. Previous studies implicate Cdc42 in regulating dynein-dependent motility. Here we show that reduced expression of the Cdc42-specific GTPase-activating protein, ARHGAP21, inhibits the ability of dispersed Golgi membranes to reposition at the centrosome following nocodazole treatment and washout. Cdc42 regulation of Golgi positioning appears to involve ARF1 and a binding interaction with the vesicle-coat protein coatomer. We tested whether Cdc42 directly affects motility, as opposed to the formation of a trafficking intermediate, using a Golgi capture and motility assay in permeabilized cells. Disrupting Cdc42 activation or the coatomer/Cdc42 binding interaction stimulated Golgi motility. The coatomer/Cdc42-sensitive motility was blocked by the addition of an inhibitory dynein antibody. Together, our results reveal that dynein and microtubule-dependent Golgi positioning is regulated by ARF1-, coatomer-, and ARHGAP21-dependent Cdc42 signaling.

Dsl1p/Zw10: common mechanisms behind tethering vesicles and microtubules

Fusion of Golgi-derived COP (coat protein)-I vesicles with the endoplasmic reticulum (ER) is initiated by specific tethering complexes: the Dsl1 (depends on SLY1-20) complex in yeast and the syntaxin 18 complex in mammalian cells. Both tethering complexes are firmly associated with soluble N-ethylmaleimide-sensitive factor attachment protein receptors (SNAREs) at the ER. The structure of the Dsl1 tethering complex has been determined recently. The complex seems to be designed to expose an unstructured domain of Dsl1p at its top, which is required to capture vesicles. The subunit composition and the interactions within the equivalent mammalian complex are similar. Interestingly, some of the mammalian counterparts have additional functions during mitosis in animal cells. Zw10, the metazoan homolog of Dsl1p, is an important component of a complex that monitors the correct tethering of microtubules to kinetochores during cell division. This review brings together evidence to suggest that there could be common mechanisms behind these different activities, giving clues as to how they might have evolved.

Identification of the neuroblastoma-amplified gene product as a component of the syntaxin 18 complex implicated in Golgi-to-endoplasmic reticulum retrograde transport

Syntaxin 18, a soluble N-ethylmaleimide-sensitive factor (NSF) attachment protein receptor (SNARE) protein implicated in endoplasmic reticulum (ER) membrane fusion, forms a complex with other SNAREs (BNIP1, p31, and Sec22b) and several peripheral membrane components (Sly1, ZW10, and RINT-1). In the present study, we showed that a peripheral membrane protein encoded by the neuroblastoma-amplified gene (NAG) is a subunit of the syntaxin 18 complex. NAG encodes a protein of 2371 amino acids, which exhibits weak similarity to yeast Dsl3p/Sec39p, an 82-kDa component of the complex containing the yeast syntaxin 18 orthologue Ufe1p. Under conditions favoring SNARE complex disassembly, NAG was released from syntaxin 18 but remained in a p31-ZW10-RINT-1 subcomplex. Binding studies showed that the extreme N-terminal region of p31 is responsible for the interaction with NAG and that the N- and the C-terminal regions of NAG interact with p31 and ZW10-RINT-1, respectively. Knockdown of NAG resulted in a reduction in the expression of p31, confirming their intimate relationship. NAG depletion did not substantially affect Golgi morphology and protein export from the ER, but it caused redistribution of Golgi recycling proteins accompanied by a defect in protein glycosylation. These results together suggest that NAG links between p31 and ZW10-RINT-1 and is involved in Golgi-to-ER transport.