On tracks and locomotives: the long route of DNA to the nucleus

Molecular motors have prominent functions in organelle transport, cytoskeletal organization, division and motility. The dyneins are one of the three families of cytoskeleton-based molecular motors and they travel along the cytoplasmic microtubule network towards the minus end of the microtubule. This directed movement is used by DNA viruses to deliver their infectious genome and proteins to the host cell nucleus. In recent studies, it has been hypothesized that Agrobacterium species use a similar pathway to deliver their infectious unit--a large complex between single-stranded DNA and proteins--to the host cell nucleus and that a karyophilic protein carrier that can deliver synthetic DNA to the nucleus is also driven by a dynein motor. These studies shed light on the mechanism of Agrobacterium-mediated genetic transformation and could lead to new methods for the efficient transfection of synthetic DNA.     

Membrane-associated DNA Transport Machines

DNA pumps play important roles in bacteria during cell division and during the transfer of genetic material by conjugation and transformation. The FtsK/SpoIIIE proteins carry out the translocation of double-stranded DNA to ensure complete chromosome segregation during cell division. In contrast, the complex molecular machines that mediate conjugation and genetic transformation drive the transport of single stranded DNA. The transformation machine also processes this internalized DNA and mediates its recombination with the resident chromosome during and after uptake, whereas the conjugation apparatus processes DNA before transfer. This article reviews these three types of DNA pumps, with attention to what is understood of their molecular mechanisms, their energetics and their cellular localizations.The transport of DNA across membranes by bacteria occurs during sporulation, during cytokinesis, directly from other cells and from the environment. This review addresses the question “how is the DNA polyanion transferred processively across the hydrophobic membrane barrier”?DNA transport must occur through water-filled channels, at least conceptually addressing the problem posed by the hydrophobic membrane. DNA transporters presumably use metabolic energy directly or a coupled-flow (symporter or antiporter) mechanism to drive DNA processively through the channel. It is possible that a Brownian ratchet mechanism, in which directionality is imposed on a diffusive process, also contributes to transport.In this article, we will consider several DNA transport systems. We will begin with the simplest one, namely the FtsK/SpoIIIE system that is involved in cell division and sporulation. We will then turn to the more complex, multiprotein DNA uptake systems that accomplish genetic transformation (the uptake of environmental DNA from the environment) and the conjugation systems of Gram-negative bacteria that mediate the unidirectional transfer of DNA between cells. In each case we will discuss the proteins involved, their actions and the sources of energy that drive transport. Space limitations prevent discussion of other relevant topics, such as DNA transport during bacteriophage infection and more than a brief reference to conjugation in Gram-positive bacteria.     

The molecular mechanism of targeted vesicle transport in cytokinesis

Recent studies have demonstrated that vesicle transport to cleavage furrow is indispensable for cytokinesis. Some animal and plant cells form distinct structures during cell division known as central spindle and phragmoplast, respectively. Several essential factors involved in the vesicle transport have been isolated so far. SNARE proteins and molecular motors play a central role in this process. For future research of cytokinesis, it is important to investigate these factors as well as cytoskeletal components of the contractile ring in detail. This review focuses on the molecular mechanism of targeted vesicle transport in cytokinesis.     

Messengers, motors and mysteries: sorting of eukaryotic mRNAs by cytoskeletal transport

It has become increasingly apparent in recent years that the subcellular localization of specific mRNAs is a prevalent method for spatially controlling gene expression. In most cases, targeting of mRNAs is mediated by transport along cytoskeletal filaments by molecular motors. However, the means by which specific messages are recognized and linked to the motors are poorly understood. Here, I will provide an overview of recent progress in elucidating the molecular mechanisms and principles of mRNA transport, including several studies highlighting the co-operation of different motors during the localization process. Important outstanding questions will also be highlighted.     

Membrane-associated nanomotors for macromolecular transport

Nature has endowed cells with powerful nanomotors to accomplish intricate mechanical tasks, such as the macromolecular transport across membranes occurring in cell division, bacterial conjugation, and in a wide variety of secretion systems. These biological motors couple the chemical energy provided by ATP hydrolysis to the mechanical work needed to transport DNA and/or protein effectors. Here, we review what is known about the molecular mechanisms of these membrane-associated machines. Sequence and structural comparison between these ATPases reveal that they share a similar motor domain, suggesting a common evolutionary ancestor. Learning how these machines operate will lead the design of nanotechnology devices with unique applications in medicine and engineering.     

Membrane trafficking during plant cytokinesis

Plant morphogenesis is regulated by cell division and expansion. Cytokinesis, the final stage of cell division, culminates in the construction of the cell plate, a unique cytokinetic membranous organelle that is assembled across the inside of the dividing cell. Both during cell-plate formation and cell expansion, the secretory pathway is highly active and is polarized toward the plane of division or toward the plasma membrane, respectively. In this review, we discuss results from recent genetic and biochemical research directed toward understanding the molecular events occurring during cytokinesis and cell expansion, including data supporting the idea that during cytokinesis one or more exocytic pathways are polarized toward the division plane. We will also highlight recent evidence for the roles of secretory vesicle transport and cytoskeletal machinery in cell-plate membrane trafficking and fusion.     

Motor proteins in cell division

The movements of eukaryotic cell division depend upon the conversion of chemical energy into mechanical work, which in turn involves the actions of motor proteins, molecular transducers that generate force and motion relative cytoskeletal elements. In animal cells, microtubule-based motor proteins of the mitotic apparatus are involved in segregating chromosomes and perhaps in organizing the mitotic apparatus itself, while microfilament-based motors in the contractile ring generate the forces that separate daughter cells during cytokinesis. This review outlines recent advances in our understanding of the roles of molecular motors in mitosis and cytokinesis.     

Viral nanomotors for packaging of dsDNA and dsRNA

While capsid proteins are assembled around single-stranded genomic DNA or RNA in rod-shaped viruses, the lengthy double-stranded genome of other viruses is packaged forcefully within a preformed protein shell. This entropically unfavourable DNA or RNA packaging is accomplished by an ATP-driven viral nanomotor, which is mainly composed of two components, the oligomerized channel and the packaging enzymes. This intriguing DNA or RNA packaging process has provoked interest among virologists, bacteriologists, biochemists, biophysicists, chemists, structural biologists and computational scientists alike, especially those interested in nanotechnology, nanomedicine, AAA+ family proteins, energy conversion, cell membrane transport, DNA or RNA replication and antiviral therapy. This review mainly focuses on the motors of double-stranded DNA viruses, but double-stranded RNA viral motors are also discussed due to interesting similarities. The novel and ingenious configuration of these nanomotors has inspired the development of biomimetics for nanodevices. Advances in structural and functional studies have increased our understanding of the molecular basis of biological movement to the point where we can begin thinking about possible applications of the viral DNA packaging motor in nanotechnology and medical applications.     

The cellular roles of molecular motors in fungi

Motors are molecular machines that move their cargo along F-actin or microtubules. Fungal representatives of myosin, kinesin and dynein motors support many cellular processes including polar growth, cell division and mitosis. Recent progress in understanding their cellular roles has revealed common principles. However, it has become obvious that fungi have also developed diverse strategies to cope with long-distance organelle transport.     

MAPK8IP1/JIP1 regulates the trafficking of autophagosomes in neurons

Autophagy is a spatially regulated process in axons; autophagosomes form preferentially in the distal axon tip then move actively and processively toward the cell body. Despite the primarily unidirectional transport observed in live-cell imaging experiments, both anterograde-directed KIF5/kinesin-1 motors and retrograde-directed dynein motors are tightly associated with axonal autophagosomes. Here, we discuss our recent work identifying the scaffolding protein MAPK8IP1/JIP1 (mitogen-activated protein kinase 8 interacting protein 1) as a key regulator of autophagosome transport in neurons. MAPK8IP1 tightly coordinates motor activity to ensure the fidelity of retrograde autophagosome transport in the axon.     

MAPK8IP1/JIP1 regulates the trafficking of autophagosomes in neurons

Autophagy is a spatially regulated process in axons; autophagosomes form preferentially in the distal axon tip then move actively and processively toward the cell body. Despite the primarily unidirectional transport observed in live-cell imaging experiments, both anterograde-directed KIF5/kinesin-1 motors and retrograde-directed dynein motors are tightly associated with axonal autophagosomes. Here, we discuss our recent work identifying the scaffolding protein MAPK8IP1/JIP1 (mitogen-activated protein kinase 8 interacting protein 1) as a key regulator of autophagosome transport in neurons. MAPK8IP1 tightly coordinates motor activity to ensure the fidelity of retrograde autophagosome transport in the axon.     

Microtubule motors at the intersection of trafficking and transport

Molecular motors drive the transport of vesicles and organelles within the cell. Traditionally, these transport processes have been considered separately from membrane trafficking events, such as regulated budding and fusion. However, recent progress has revealed mechanistic links that integrate these processes within the cell. Rab proteins, which function as key regulators of intracellular trafficking, have now been shown to recruit specific motors to organelle membranes. Rab-independent recruitment of motors by adaptor or scaffolding proteins is also a key mechanism. Once recruited to vesicles and organelles, these motors can then drive directed transport; this directed transport could in turn affect the efficiency of trafficking events. Here, we discuss this coordinated regulation of trafficking and transport, which provides a powerful mechanism for temporal and spatial control of cellular dynamics.     

分子发动机研究进展

分子发动机是利用化学能/化学势进行机械作功的生物大分子,包括线性分子发动机与旋转式分子发动机两大类.它们参与了胞质运输、DNA复制、基因转录、ATP合成／水解等一系列重要生命活动过程.目前对于各种分子发动机的结构及作用机制的研究取得了一些重要进展.     

A single calcium flux triggers chromosome replication, segregation and septation in bacteria: a model

Abrupt changes in the concentration of intracellular calcium, through the mediation of calmodulin, is presumed to play an essential role in many molecular processes in eukaryotes including triggering cell cycle events. Although early studies failed to establish any role for calcium in the growth of bacteria, recent studies have demonstrated that bacteria have several calcium transport systems, and an intracellular concentration of free calcium identical to that of higher organisms, which appears to fluctuate during the cell cycle. Moreover, calmodulin-like proteins have been reported in bacteria, and the growth of E. coli is sensitive to calmodulin inhibitors. In this article we propose that a single flux of calcium, abruptly raising the intracellular concentration of free calcium, is responsible for the triggering in bacteria of the major cell cycle events, initiation of DNA replication, chromosome partition and cell division. We predict that major roles in this process will involve a bacterial calmodulin-like protein and a primitive cytoskeleton. The mechanism of triggering different cell cycle events by a single calcium flux is discussed.     

Molecular cloning and functional analysis of mouse C-terminal kinesin motor KifC3

Proteins of the kinesin superfamily define a class of microtubule-dependent motors that play crucial roles in cell division and intracellular transport. To study the molecular mechanism of intracellular transport involving microtubule-dependent motors, a cDNA encoding a new kinesin-like protein called KifC3 was cloned from a mouse brain cDNA library. Sequence and secondary structure analysis revealed that KifC3 is a member of the C-terminal motor family. In contrast to other mouse C-terminal motors, KifC3 is apparently ubiquitous and may have a general role in intracellular transport. To understand the in vivo function of the KifC3 gene, we used homologous recombination in embryonic stem cells to construct knockout mouse strains for the KifC3 gene. Homozygous mutants of the KifC3 gene are viable, reproduce normally, and apparently develop normally. These results suggest that KifC3 is dispensable for normal development and reproduction in the mouse.     

Kinesin 5B (KIF5B) Is Required for Progression through Female Meiosis and Proper Chromosomal Segregation in Mitotic Cells

The fidelity of chromosomal segregation during cell division is important to maintain chromosomal stability in order to prevent cancer and birth defects. Although several spindle-associated molecular motors have been shown to be essential for cell division, only a few chromosome arm-associated motors have been described. Here, we investigated the role of Kinesin 5b (Kif5b) during female mouse meiotic cell development and mitotic cell division. RNA interference (RNAi)-mediated silencing of Kif5b in mouse oocytes induced significant delay in germinal vesicle breakdown (GVBD) and failure in extrusion of the first polar body (PBE). In mitotic cells, knockdown of Kif5b leads to centrosome amplification and a chromosomal segregation defect. These data suggest that KIF5B is critical in suppressing chromosomal instability at the early stages of female meiotic cell development and mitotic cell division.     

Elucidating the molecular mechanisms underlying cellular response to biophysical cues using synthetic biology approaches

ABSTRACT

The use of synthetic surfaces and materials to influence and study cell behavior has vastly progressed our understanding of the underlying molecular mechanisms involved in cellular response to physicochemical and biophysical cues. Reconstituting cytoskeletal proteins and interfacing them with a defined microenvironment has also garnered deep insight into the engineering mechanisms existing within the cell. This review presents recent experimental findings on the influence of several parameters of the extracellular environment on cell behavior and fate, such as substrate topography, stiffness, chemistry and charge. In addition, the use of synthetic environments to measure physical properties of the reconstituted cytoskeleton and their interaction with intracellular proteins such as molecular motors is discussed, which is relevant for understanding cell migration, division and structural integrity, as well as intracellular transport. Insight is provided regarding the next steps to be taken in this interdisciplinary field, in order to achieve the global aim of artificially directing cellular response.