Two Saccharomyces cerevisiae kinesin-related gene products required for mitotic spindle assembly

Pds1p, an inhibitor of anaphase in budding yeast, plays a critical role in the APC and checkpoint pathway(s)

We report the isolation and characterization of pds1 mutants in Saccharomyces cerevisiae. The initial pds1-1 allele was identified by its inviability after transient exposure to microtubule inhibitors and its precocious dissociation of sister chromatids in the presence of these microtubule inhibitors. These findings suggest that pds1 mutants might be defective in anaphase arrest that normally is imposed by a spindle-damage checkpoint. To further examine a role for Pds1p in anaphase arrest, we compared the cell cycle arrest of pds1 mutants and PDS1 cells after: (a) the inactivation of Cdc16p or Cdc23p, two proteins that are required for the degradation of mitotic cyclins and are putative components of the yeast anaphase promoting complex (APC); (b) the inactivation of Cdc20p, another protein implicated in the degradation of mitotic cyclins; and (c) the inactivation of Cdc13 protein or gamma irradiation, two circumstances that induce a DNA-damage checkpoint. Under all these conditions, anaphase is inhibited in PDS1 cells but not in pds1 mutants. From these results we suggest that Pds1 protein is an anaphase inhibitor in PDS1 cells but not in pds1 mutants. From these results we suggest that Pds1 protein is an anaphase inhibitor that plays a critical role in the control of anaphase by both APC and checkpoints. We also show that pds1 mutants exit mitosis and initiate new rounds of cell division after gamma irradiation and Cdc13p inactivation but no after nocodazole-treatment or inactivation of Cdc16p, Cdc20p or Cdc23p function. Therefore, in the DNA-damage checkpoint, Pds1p is required for the inhibition of cytokinesis and DNA replication as well as anaphase. The role of Pds1 protein in anaphase inhibition and general cell cycle regulation is discussed.

Going mobile: microtubule motors and chromosome segregation

Proper chromosome segregation in eukaryotes depends upon the mitotic and meiotic spindles, which assemble at the time of cell division and then disassemble upon its completion. These spindles are composed in large part of microtubules, which either generate force by controlled polymerization and depolymerization or transduce force generated by molecular microtubule motors. In this review, we discuss recent insights into chromosome segregation mechanisms gained from the analyses of force generation during meiosis and mitosis. These analyses have demonstrated that members of the kinesin superfamily and the dynein family are essential in all organisms for proper chromosome and spindle behavior. It is also apparent that forces generated by microtubule polymerization and depolymerization are capable of generating forces sufficient for chromosome movement in vitro; whether they do so in vivo is as yet unclear. An important realization that has emerged is that some spindle activities can be accomplished by more than one motor so that functional redundancy is evident. In addition, some meiotic or mitotic movements apparently occur through the cooperative action of independent semiredundant processes. Finally, the molecular characterization of kinesin-related proteins has revealed that variations both in primary sequence and in associations with other proteins can produce motor complexes that may use a variety of mechanisms to transduce force in association with microtubules. Much remains to be learned about the regulation of these activities and the coordination of opposing and cooperative events involved in chromosome segregation; this set of problems represents one of the most important future frontiers of research.

Kinesin proteins: a phylum of motors for microtubule-based motility

The cellular processes of transport, division and, possibly, early development all involve microtubule-based motors. Recent work shows that, unexpectedly, many of these cellular functions are carried out by different types of kinesin and kinesin-related motor proteins. The kinesin proteins are a large and rapidly growing family of microtubule-motor proteins that share a 340-amino-acid motor domain. Phylogenetic analysis of the conserved motor domains groups the kinesin proteins into a number of subfamilies, the members of which exhibit a common molecular organization and related functions. The kinesin proteins that belong to different subfamilies differ in their rates and polarity of movement along microtubules, and probably in the particles/organelles that they transport. The kinesins arose early in eukaryotic evolution and gene duplication has allowed functional specialization to occur, resulting in a surprisingly large number of different classes of these proteins adapted for intracellular transport of vesicles and organelles, and for assembly and force generation in the meiotic and mitotic spindles.

The kinesin-homologous protein encoded by the Chlamydomonas FLA10 gene is associated with basal bodies and centrioles

We previously reported that the FLA10 locus on the uni linkage group of Chlamydomonas encodes a kinesin homologous protein, KHP1. The fla10 phenotype, which is a temperature-sensitive defect for flagellar assembly and maintenance, is rescued by transformation with the wild-type KHP1 gene. In the present study we identify the molecular defect associated with the fla10 mutation and examine the subcellular localization of KHP1 throughout the cell cycle. The mutation in the fla10-1 allele consists of a C to A transversion, which alters amino acid 329 in the motor domain of KHP1. This residue and the sequence of the carboxy-terminal third of the motor domain in which it is located are highly conserved throughout eukaryotic evolution in a subfamily of kinesin-related proteins from mouse (KIF3), sea urchin (KRP85/95), Xenopus (XKLP3), and Drosophila (KLP68D). These data suggest a conserved function for this family of proteins. Immunofluorescence studies reveal that: (1) in interphase cells KHP1 is associated with basal bodies and with the proximal portion of the flagella; (2) in cells undergoing flagellar regeneration KHP1 occurs in punctate structures that extend to the tip of the developing axoneme; and (3) in dividing cells KHP1 remains associated with centrioles throughout mitosis and localizes to the mitotic spindle. KHP1 is the first kinesin homologous protein to be found in association with basal bodies and centrioles throughout the cell cycle. These observations provide evidence for a direct role of basal bodies in the process of flagellar development, which we propose is based on KHP1 acting as a transporter of flagellar components from the basal bodies out to the distal site of assembly. The localization of KHP1 in mitosis suggests that this protein may play an analogous role in the centriole-based assembly of the mitotic spindle.

Kinesin-related proteins in the mammalian testes: candidate motors for meiosis and morphogenesis

The kinesin superfamily of molecular motors comprises proteins that participate in a wide variety of motile events within the cell. Members of this family share a highly homologous head domain responsible for force generation attached to a divergent tail domain thought to couple the motor domain to its target cargo. Many kinesin-related proteins (KRPs) participate in spindle morphogenesis and chromosome movement in cell division. Genetic analysis of mitotic KRPs in yeast and Drosophila, as well as biochemical experiments in other species, have suggested models for the function of KRPs in cell division, including both mitosis and meiosis. Although many mitotic KRPs have been identified, the relationship between mitotic motors and meiotic function is not clearly understood. We have used sequence similarity between mitotic KRPs to identify candidates for meiotic and/or mitotic motors in a vertebrate. We have identified a group of kinesin-related proteins from rat testes (termed here testes KRP1 through KRP6) that includes new members of the bimC and KIF2 subfamilies as well as proteins that may define new kinesin subfamilies. Five of the six testes KRPs identified are expressed primarily in testes. Three of these are expressed in a region of the seminiferous epithelia (SE) rich in meiotically active cells. Further characterization of one of these KRPs, KRP2, showed it to be a promising candidate for a motor in meiosis: it is localized to a meiotically active region of the SE and is homologous to motor proteins associated with the mitotic apparatus. Testes-specific genes provide the necessary probes to investigate whether the motor proteins that function in mammalian meiosis overlap with those of mitosis and whether motor proteins exist with functions unique to meiosis. Our search for meiotic motors in a vertebrate testes has successfully identified proteins with properties consistent with those of meiotic motors in addition to uncovering proteins that may function in other unique motile events of the SE.

Motors involved in spindle assembly and chromosome segregation

During the past two years, major advances have been made in our understanding of the role of motor proteins in chromosome-microtubule interactions in the spindle. The discovery of kinesin-like proteins (KLPs) associated with chromosome arms has shed some light on the mechanism of chromosome congression and the establishment of spindle bipolarity. Recent results also indicate that kinetochore KLPs may tether the ends of growing and shrinking microtubules to kinetochores during chromosome movements. Finally, new data indicate that phosphorylation of KLPs may be one of the mechanisms by which they are targeted to specific spindle domains.

Kid, a novel kinesin-like DNA binding protein, is localized to chromosomes and the mitotic spindle

Microtubule-associated motor proteins are thought to be involved in spindle formation and chromosome movements in mitosis/meiosis. We have molecularly cloned cDNAs for a gene that codes for a novel member of the kinesin family of proteins. Nucleotide sequencing reveals that the predicted gene product is a 73 kDa protein and is related to some extent to the Drosophila node gene product, which is involved in chromosomal segregation during meiosis. A sequence similar to the microtubule binding motor domain of kinesin is present in the N-terminal half of the protein, and its ability to bind to microtubules is demonstrated. Furthermore we show that its C-terminal half contains a putative nuclear localization signal similar to that of Jun and is able to bind to DNA. Accordingly, the protein was termed Kid (kinesin-like DNA binding protein). Indirect immunofluorescence studies show that Kid colocalizes with mitotic chromosomes and that it is enriched in the kinetochore at anaphase. Thus, we propose that Kid might play a role(s) in regulating the chromosomal movement along microtubules during mitosis.

A bipolar kinesin

Chromosome segregation during mitosis depends on the action of the mitotic spindle, a self-organizing, bipolar protein machine which uses microtubules (MTs) and their associated motors. Members of the BimC subfamily of kinesin-related MT-motor proteins are believed to be essential for the formation and functioning of a normal bipolar spindle. Here we report that KRP130, a homotetrameric BimC-related kinesin purified from Drosophila melanogaster embryos, has an unusual ultrastructure. It consists of four kinesin-related polypeptides assembled into a bipolar aggregate with motor domains at opposite ends, analogous to a miniature myosin filament. Such a bipolar 'minifilament' could crosslink spindle MTs and slide them relative to one another. We do not know of any other MT motors that have a bipolar structure.

Role of gamma-tubulin in mitosis-specific microtubule nucleation from the Schizosaccharomyces pombe spindle pole body

The ability of the Schizosacchromyces pombe spindle pole body to nucleate microtubules is activated at the onset of mitosis for forming a mitotic spindle, but it is inactivated during interphase. We have previously developed an in vitro assay for studying the molecular mechanism of spindle pole body activation using permeabilized interphase S. pombe cells and Xenopus mitotic extracts. We have shown that the interphase spindle pole body is activated indirectly by p34cdc2 protein kinase in Xenopus mitotic extracts. In this study we examined the role of gamma-tubulin, a component of both interphase and mitotic spindle pole body, in formation of the microtubule nucleating complex at the mitotic spindle pole body. A polyclonal antibody specific to S. pombe gamma-tubulin inhibited both activation of the interphase spindle pole body and microtubule nucleation from the mitotic spindle pole body. Addition of bacterially expressed S. pombe gamma-tubulin or its amino-terminal fragments to Xenopus mitotic extracts inhibited spindle pole body activation. Affinity chromatography of partially fractionated Xenopus mitotic extracts with the amino-terminal fragment of S. pombe gamma-tubulin showed that fractions bound to the fragment supported the activation. The fractions did not contain Xenopus gamma-tubulin, showing that activation of the spindle pole body is not due to recruitment of Xenopus gamma-tubulin to the spindle pole body. The spindle pole body activation occurred in extracts depleted of p34cdc2 protein kinase or MAP kinase. The activity of the fractions bound to the fragment was inhibited by a protein kinase inhibitor, staurosporine. These results suggest that S. pombe gamma-tubulin is a component of the microtubule nucleating complex, and that the function of proteins that interact with gamma-tubulin is required for activation of the spindle pole body. We present possible models for the activation that convert the immature microtubule nucleating complex at interphase into the mature microtubule nucleating complex at mitosis.

The centrosome in animal cells and its functional homologs in plant and yeast cells

The centrosome is the principal microtubule-organizing center in mammalian cells. Until recently, the centrosome could only be studied at the ultrastructural level and defined as a functional entity. However, during the past decade a number of clever experimental strategies have been used to identify numerous molecular components of the centrosome. The identification of biochemical subunits of the centrosome complex has allowed the centrosome to be investigated in much more detail, resulting in important advances being made in our understanding of microtubule nucleation events, spindle formation, the assembly and replication of the centrosome, and the nature of the microtubule-organizing centers in plant cells and lower eukaryotes. The next several years should see additional rapid progress in our understanding of the microtubule cytoskeleton as investigators begin to assign functions to the centrosome proteins that have already been reported and as additional centrosome components are discovered.

Microtubules and microtubule motors: mechanisms of regulation

Microtubule-based motility is precisely regulated, and the targets of regulation may be the motor proteins, the microtubules, or both components of this intricately controlled system. Regulation of microtubule behavior can be mediated by cell cycle-dependent changes in centrosomal microtubule nucleating ability and by cell-specific, microtubule-associated proteins (MAPs). Changes in microtubule organization and dynamics have been correlated with changes in phosphorylation. Regulation of motor proteins may be required both to initiate movement and to dictate its direction. Axonemal and cytoplasmic dyneins as well as kinesin can be phosphorylated and this modification may affect the motor activities of these enzymes or their ability to interact with organelles. A more complete understanding of how motors can be modulated by phosphorylation, either of the motor proteins or of other associated substrates, will be necessary in order to understand how bidirectional transport is regulated.

Phosphorylation by p34cdc2 regulates spindle association of human Eg5, a kinesin-related motor essential for bipolar spindle formation in vivo

We have isolated a human homolog of Xenopus Eg5, a kinesin-related motor protein implicated in the assembly and dynamics of the mitotic spindle. We report that microinjection of antibodies against human Eg5 (HsEg5) blocks centrosome migration and causes HeLa cells to arrest in mitosis with monoastral microtubule arrays. Furthermore, an evolutionarily conserved cdc2 phosphorylation site (Thr-927) in HsEg5 is phosphorylated specifically during mitosis in HeLa cells and by p34cdc2/cyclin B in vitro. Mutation of Thr-927 to nonphosphorylatable residues prevents HsEg5 from binding to centrosomes, indicating that phosphorylation controls the association of this motor with the spindle apparatus. These results indicate that HsEg5 is required for establishing a bipolar spindle and that p34cdc2 protein kinase directly regulates its localization.

Two genes required for the binding of an essential Saccharomyces cerevisiae kinetochore complex to DNA

Kinetochores are DNA-protein structures that assemble on centromeric DNA and attach chromosomes to spindle microtubules. Because of their simplicity, the 125-bp centromeres of Saccharomyces cerevisiae are particularly amenable to molecular analysis. Budding yeast centromeres contain three sequence elements of which centromere DNA sequence element III (CDEIII) appears to be particularly important. cis-acting mutations in CDEIII and trans-acting mutations in genes encoding subunits of the CDEIII-binding complex (CBF3) prevent correct chromosome transmission. Using temperature-sensitive mutations in CBF3 subunits, we show a strong correlation between DNA-binding activity measured in vitro and kinetochore activity in vivo. We extend previous findings by Goh and Kilmartin [Goh, P.-Y. & Kilmartin, J.V. (1993) J. Cell Biol. 121, 503-512] to argue that DNA-bound CBF3 may be involved in the operation of a mitotic checkpoint but that functional CBF3 is not required for the assembly of a bipolar spindle.

A highly divergent gamma-tubulin gene is essential for cell growth and proper microtubule organization in Saccharomyces cerevisiae

A Saccharomyces cerevisiae gamma-tubulin-related gene, TUB4, has been characterized. The predicted amino acid sequence of the Tub4 protein (Tub4p) is 29-38% identical to members of the gamma-tubulin family. Indirect immunofluorescence experiments using a strain containing an epitope-tagged Tub4p indicate that Tub4p resides at the spindle pole body throughout the yeast cell cycle. Deletion of the TUB4 gene indicates that Tub4p is essential for yeast cell growth. Tub4p-depleted cells arrest during nuclear division; most arrested cells contain a large bud, replicated DNA, and a single nucleus. Immunofluorescence and nuclear staining experiments indicate that cells depleted of Tub4p contain defects in the organization of both cytoplasmic and nuclear microtubule arrays; such cells exhibit nuclear migration failure, defects in spindle formation, and/or aberrantly long cytoplasmic microtubule arrays. These data indicate that the S. cerevisiae gamma-tubulin protein is an important SPB component that organizes both cytoplasmic and nuclear microtubule arrays.

Yeast motor proteins

Yeast accomplish a variety of intracellular motile events with the aid of mechanochemical enzymes known as motor proteins. This review covers the current state of knowledge on myosins, kinesins, dyneins, dynamins and SMC proteins present in yeast cells, and the most important developments in the study of yeast mitosis. Both topics have seen rapid progress over the past few years.

The Neurospora organelle motor: a distant relative of conventional kinesin with unconventional properties

The "conventional" kinesins comprise a conserved family of molecular motors for organelle transport that have been identified in various animal species. Organelle motors from other phyla have not yet been analyzed at the molecular level. Here we report the identification, biochemical and immunological characterization, and molecular cloning of a cytoplasmic motor in a "lower" eukaryote, the Ascomycete fungus Neurospora crassa. This motor, termed Nkin (for Neurospora kinesin), exhibits several unique structural and functional features, including a high rate of microtubule transport, a lack of copurifying light chains, a second P-loop motif, and an overall sequence organization reminiscent of a kinesin-like protein. However, a greater than average sequence homology in the motor domain and the presence of a highly conserved region in the C-terminus identify Nkin as a distant relative of the family of conventional kinesins. A molecular phylogenetic analysis suggests Nkin to have diverged early in the evolution of this family of motors. The discovery of Nkin may help identify domains important for specific biological functions in conventional kinesins.

Motor activity and mitotic spindle localization of the Drosophila kinesin-like protein KLP61F

The KLP61F gene product is essential for Drosophila development. Mutations in KLP61F display a mitotic arrest phenotype caused by a failure in the proper separation of duplicated centrosomes (Heck et al., 1993). Sequence analysis of KLP61F identified it as a member of the bimC family of kinesin-like microtubule motor proteins. Here we report that KLP61F is distinct from KRP130, a kinesin-like protein recently purified from Drosophila embryos and suggested to be the product of the KLP61F gene (Cole et al., 1994). We also characterized recombinant KLP61F and found that it possesses microtubule-stimulated ATPase and microtubule translocation activities in vitro. In addition, we have used an affinity-purified, KLP61F-specific antiserum to localize native KLP61F and an epitope-tagged KLP61F fusion protein during various stages of mitosis in Drosophila syncytial blastoderm embryos. From early prophase through anaphase, KLP61F is coincident with the distribution of tubulin. Together these results confirm the existence of multiple bimC-like kinesins in Drosophila and suggest that KLP61F function is intrinsic to the mitotic spindle.

Kinetics of spindle pole body separation in budding yeast

In the budding yeast Saccharomyces cerevisiae, the spindle pole body (SPB) serves as the microtubule-organizing center and is the functional analog of the centrosome of higher organisms. By expressing a fusion of a yeast SPB-associated protein to the Aequorea victoria green fluorescent protein, the movement of the SPBs in living yeast cells undergoing mitosis was observed by fluorescence microscopy. The ability to visualize SPBs in vivo has revealed previously unidentified mitotic events. During anaphase, the mitotic spindle has four sequential activities: alignment at the mother-daughter junction, fast elongation, translocation into the bud, and slow elongation. These results indicate that distinct forces act upon the spindle at different times during anaphase.

Two microtubule-associated proteins required for anaphase spindle movement in Saccharomyces cerevisiae

In many eucaryotic cells, the midzone of the mitotic spindle forms a distinct structure containing a specific set of proteins. We have isolated ASE1, a gene encoding a component of the Saccharomyces cerevisiae spindle midzone. Strains lacking both ASE1 and BIK1, which encodes an S. cerevisiae microtubule-associated protein, are inviable. The analysis of the phenotype of a bik1 ase1 conditional double mutant suggests that BIK1 and ASE1 are not required for the assembly of a bipolar spindle, but are essential for anaphase spindle elongation. The steady-state levels of Ase1p are regulated in a manner that is consistent with a function during anaphase: they are low in G1, accumulate to maximal levels after S phase and then drop as cells exit mitosis. Components of the spindle midzone may therefore be required in vivo for anaphase spindle movement. Additionally, anaphase spindle movement may depend on a dedicated set of genes whose expression is induced at G2/M.

Microtubule stability in budding yeast: characterization and dosage suppression of a benomyl-dependent tubulin mutant

To better understand the dynamic regulation of microtubule structures in yeast, we studied a conditional-lethal beta-tubulin mutation tub2-150. This mutation is unique among the hundreds of tubulin mutations isolated in Saccharomyces cerevisiae in that it appears to cause an increase in the stability of microtubules. We report here that this allele is a mutation of threonine 238 to alanine, and that tub2-150 prevents the spindle from elongating during anaphase, suggesting a nuclear microtubule defect. To identify regulators of microtubule stability and/or anaphase, yeast genes were selected that, when overexpressed, could suppress the tub2-150 temperature-sensitive phenotype. One of these genes, JSN1, encodes a protein of 125 kDa that has limited similarity to a number of proteins of unknown function. Overexpression of the JSN1 gene in a TUB2 strain causes that strain to become more sensitive to benomyl, a microtubule-destabilizing drug. Of a representative group of microtubule mutants, only one other mutation, tub2-404, could be suppressed by JSN1 overexpression, showing that JSN1 is an allele-specific suppressor. As tub2-404 mutants are also defective for spindle elongation, this provides additional support for a role for JSN1 during anaphase.

Yeast Srp1, a nuclear protein related to Drosophila and mouse pendulin, is required for normal migration, division, and integrity of nuclei during mitosis

This paper describes genes from yeast and mouse with significant sequence similarities to a Drosophila gene that encodes the blood cell tumor suppressor pendulin. The protein encoded by the yeast gene, Srp1p, and mouse pendulin share 42% and 51% amino acid identity with Drosophila pendulin, respectively. All three proteins consist of 10.5 degenerate tandem repeats of approximately 42 amino acids each. Similar repeats occur in a superfamily of proteins that includes the Drosophila Armadillo protein. All three proteins contain a consensus sequence for a bipartite nuclear localization signal (NLS) in the N-terminal domain, which is not part of the repeat structure. Confocal microscopic analysis of yeast cells stained with antibodies against Srp1p reveals that this protein is intranuclear throughout the cell cycle. Targeted gene disruption shows that SRP1 is an essential gene. Despite their sequence similarities, Drosophila and mouse pendulin are unable to rescue the lethality of an SRP1 disruption. We demonstrate that yeast cells depleted of Srp1p arrest in mitosis with a G2 content of DNA. Arrested cells display abnormal structures and orientations of the mitotic spindles, aberrant segregation of the chromatin and the nuclei, and threads of chromatin emanating from the bulk of nuclear DNA. This phenotype suggests that Srp1p is required for the normal function of microtubules and the spindle pole bodies, as well as for nuclear integrity. We suggest that Srp1p interacts with multiple components of the cell nucleus that are required for mitosis and discuss its functional similarities to, and differences from Drosophila pendulin.

Is there a unique form of chromatin at the Saccharomyces cerevisiae centromeres?

Chromosome transmission in S. cerevisiae requires the activities of many structural and regulatory proteins required for the replication, repair, recombination and segregation of chromosomal DNA, and co-ordination of the chromosome cycle with progression through cell cycle. An important structural domain on each chromosome is the kinetochore (centromere DNA and associated proteins), which provides the site of attachment of chromosomes to the spindle microtubules. Stoler et al. have recently reported the cloning of an essential gene CSE4, mutations in which cause chromosome nondisjunction of a marked chromosome bearing a centromere DNA mutation. The cse4-1 mutation causes cells to arrest in the G2/M phase of the cell cycle with a 2N DNA content in a RAD9 checkpoint-independent manner. The carboxyl terminus of Cse4p and the human centromere-localized protein CENP-A have a high degree of homology to the C-terminal domain of histone H3. Since both CENP-A and Cse4p also have biochemical properties similar to histones H3 and H4, it is tempting to speculate that these histone H3-like proteins are components of specialized nucleosomes, a class of which may be unique to the centromeres.

Disturbance of normal cell cycle progression enhances the establishment of transcriptional silencing in Saccharomyces cerevisiae

Previous studies have indicated that mutation of RAP1 (rap1s) or of the HMR-E silencer ARS consensus element leads to metastable repression of HMR. A number of extragenic suppressor mutations (sds, suppressors of defective silencing) that increase the fraction of repressed cells in rap1s hmr delta A strains have been identified. Here we report the cloning of three SDS genes. SDS11 is identical to SWI6, a transcriptional regulator of genes required for DNA replication and of cyclin genes. SDS12 is identical to RNR1, which encodes a subunit of ribonucleotide reductase. SDS15 is identical to CIN8, whose product is required for spindle formation. We propose that mutations in these genes improve the establishment of silencing by interfering with normal cell cycle progression. In support of this idea, we show that exposure to hydroxyurea, which increases the length of S phase, also restores silencing in rap1s hmr delta A strains. Mutations in different cyclin genes (CLN3, CLB5, and CLB2) and two cell cycle transcriptional regulators (SWI4 and MBP1) also suppress the silencing defect at HMR. The effect of these cell cycle regulators is not specific to the rap1s or hmr delta A mutation, since swi6, swi4, and clb5 mutations also suppress mutations in SIR1, another gene implicated in the establishment of silencing. Several mutations also improve the efficiency of telomeric silencing in wild-type strains, further demonstrating that disturbance of the cell cycle has a general effect on position effect repression in Saccharomyces cerevisiae. We suggest several possible models to explain this phenomenon.

Frontier questions about sister chromatid separation in anaphase

Sister chromatid separation in anaphase is an important event in the cell's transmission of genetic information to a descendent. It has been investigated from different aspects: cell cycle regulation, spindle and chromosome dynamics within the three-dimensional cell architecture, transmission fidelity control and cellular signaling. Integrated studies directed toward unified understanding are possible using multidisciplinary methods with model organisms. Ubiquitin-dependent proteolysis, protein dephosphorylation, an unknown function by the TPR repeat proteins, chromosome transport by microtubule-based motors and DNA topological change by DNA topoisomerase II are all necessary for progression from metaphase to anaphase. Chromosome condensation, mitotic kinetochore function and spindle formation require a larger number of proteins, which are prerequisites for successful sister chromatid separation. Factors that help to retain sister chromatid connection after replication and prevent premature separation remain to be determined. Although sister chromatid separation occurs in anaphase, gene functions in other cell cycle stages also ensure the progression of correct chromatid separation.

Mutations in the kinesin-like protein Eg5 disrupting localization to the mitotic spindle

Eg5, a member of the bimC subfamily of kinesin-like microtubule motor proteins, localizes to spindle microtubules in mitosis but not to interphase microtubules. We investigated the molecular basis for spindle localization by transient transfection of Xenopus A6 cells with myc-tagged derivatives of Eg5. Expressed at constitutively high levels from a cytomegalovirus promoter, mycEg5 protein is cytoplasmic throughout interphase, begins to bind microtubules in early prophase, and remains localized to spindle and/or midbody microtubules through mitosis to the end of telophase. Both N- and C-terminal regions of Eg5 are required for this cell-cycle-regulated targeting. Eg5 also contains within its C-terminal domain a sequence conserved among bimC subfamily proteins that includes a potential p34cdc2 phosphorylation site. We show that mutation of a single threonine (T937) within this site to nonphosphorylatable alanine abolishes localization of the mutant protein to the spindle, whereas mutation of T937 to serine preserves spindle localization. We hypothesize that phosphorylation of Eg5 may regulate its localization to the spindle in the cell cycle.

The product of the spindle formation gene sad1+ associates with the fission yeast spindle pole body and is essential for viability

Spindle formation in fission yeast occurs by the interdigitation of two microtubule arrays extending from duplicated spindle pole bodies which span the nuclear membrane. By screening a bank of temperature-sensitive mutants by anti-tubulin immunofluorescence microscopy, we previously identified the sad1.1 mutation (Hagan, I., and M. Yanagida. 1990. Nature (Lond.). 347:563-566). Here we describe the isolation and characterization of the sad1+ gene. We show that the sad1.1 mutation affected both spindle formation and function. The sad1+ gene is a novel essential gene that encodes a protein with a predicted molecular mass of 58 kD. Deletion of the gene was lethal resulting in identical phenotypes to the sad1.1 mutation. Sequence analysis predicted a potential membrane-spanning domain and an acidic amino terminus. Sad1 protein migrated as two bands of 82 and 84 kD on SDS-PAGE, considerably slower than its predicted mobility, and was exclusively associated with the spindle pole body (SPB) throughout the mitotic and meiotic cycles. Microtubule integrity was not required for Sad1 association with the SPB. Upon the differentiation of the SPB in metaphase of meiosis II, Sad1-staining patterns similarly changed from a dot to a crescent supporting an integral role in SPB function. Moderate overexpression of Sad1 led to association with the nuclear periphery. As Sad1 was not detected in the cytoplasmic microtubule-organizing centers activated at the end of anaphase or kinetochores, we suggest that Sad1 is not a general component of microtubule-interacting structures per se, but is an essential mitotic component that associates with the SPB but is not required for microtubule nucleation. Sad1 may play a role in SPB structure, such as maintaining a functional interface with the nuclear membrane or in providing an anchor for the attachment of microtubule motor proteins.

Characterization of a minus end-directed kinesin-like motor protein from cultured mammalian cells

Using the CHO2 monoclonal antibody raised against CHO spindles (Sellitto, C., M. Kimble, and R. Kuriyama. 1992. Cell Motil. Cytoskeleton. 22:7-24) we identified a 66-kD protein located at the interphase centrosome and mitotic spindle. Isolated cDNAs for the antigen encode a 622-amino acid polypeptide. Sequence analysis revealed the presence of 340-amino acid residues in the COOH terminus, which is homologous to the motor domain conserved among other members of the kinesin superfamily. The protein is composed of a central alpha-helical portion with globular domains at both NH2 and COOH termini, and the epitope to the monoclonal antibody resides in the central alpha-helical stalk. A series of deletion constructs were created for in vitro analysis of microtubule interactions. While the microtubule binding and bundling activities require both the presence of the COOH terminus and the alpha-helical domain, the NH2-terminal half of the antigen lacked the ability to interact with microtubules. The full-length as well as deleted proteins consisting of the COOH-terminal motor and the central alpha-helical stalk supported microtubule gliding, with velocity ranging from 1.0 to 8.4 microns/minute. The speed of microtubule movement decreased with decreasing lengths of the central stalk attached to the COOH-terminal motor. The microtubules moved with their plus end leading, indicating that the antigen is a minus end-directed motor. The CHO2 sequence shows 86% identify to HSET, a gene located at the centromeric end of the human MHC region in chromosome 6 (Ando, A., Y. Y. Kikuti, H. Kawata, N. Okamoto, T. Imai, T. Eki, K. Yokoyama, E. Soeda, T. Ikemura, K. Abe, and H. Inoko. 1994. Immunogenetics. 39:194-200), indicating that HSET might represent a human homologue of the CHO2 antigen.

The Drosophila kinesin-like protein KLP3A is a midbody component required for central spindle assembly and initiation of cytokinesis

We describe here a new member of the kinesin superfamily in Drosophila, KLP3A (Kinesin-Like-Protein-at-3A). The KLP3A protein localizes to the equator of the central spindle during late anaphase and telophase of male meiosis. Mutations in the KLP3A gene disrupt the interdigitation of microtubules in spermatocyte central spindles. Despite this defect, anaphase B spindle elongation is not obviously aberrant. However, cytokinesis frequently fails after both meiotic divisions in mutant testes. Together, these findings strongly suggest that the KLP3A presumptive motor protein is a critical component in the establishment or stabilization of the central spindle. Furthermore, these results imply that the central spindle is the source of signals that initiate the cleavage furrow in higher cells.

Sgs1: a eukaryotic homolog of E. coli RecQ that interacts with topoisomerase II in vivo and is required for faithful chromosome segregation

Topoisomerase II (topo II) catalyzes the decatenation of interlinked DNA molecules and is essential for chromosome segregation. To test the hypothesis that the noncatalytic C-terminal domain of topo II is necessary for mediating interactions with other proteins required for chromosome segregation, we used a two-hybrid cloning strategy to identify proteins that interact with S. cerevisiae topo II in vivo. One protein identified (Sgs1p) is structurally related to E. coli RecQ protein and contains helicase signature motifs. Strains lacking Sgs1p exhibit elevated levels of chromosome misseggregation during both mitotic and meiotic division. We propose a model to account for the interaction of a topoisomerase and a helicase in the faithful segregation of newly replicated eukaryotic chromosomes.

A second nitrogen permease regulator in Saccharomyces cerevisiae

We describe a Saccharomyces cerevisiae mutant affected in its urea and proline transport capacities, and a gene coding for a protein complementing this mutation. This protein is not membrane-embedded and contains two PEST sequences, often found in regulatory factors. The mRNA is not down-regulated under nitrogen catabolite repression, and is induced by urea and proline. In the mutant, the PUT4 mRNA encoding the proline permease is not affected, whereas the DUR3 mRNA, involved in urea active transport, is strongly increased. Our data suggest that this protein is a post-transcriptional regulator of nitrogen permeases.

Saccharomyces cerevisiae kinesin- and dynein-related proteins required for anaphase chromosome segregation

The Saccharomyces cerevisiae kinesin-related gene products Cin8p and Kip1p function to assemble the bipolar mitotic spindle. The cytoplasmic dynein heavy chain homologue Dyn1p (also known as Dhc1p) participates in proper cellular positioning of the spindle. In this study, the roles of these motor proteins in anaphase chromosome segregation were examined. While no single motor was essential, loss of function of all three completely halted anaphase chromatin separation. As combined motor activity was diminished by mutation, both the velocity and extent of chromatin movement were reduced, suggesting a direct role for all three motors in generating a chromosome-separating force. Redundancy for function between different types of microtubule-based motor proteins was also indicated by the observation that cin8 dyn1 double-deletion mutants are inviable. Our findings indicate that the bulk of anaphase chromosome segregation in S. cerevisiae is accomplished by the combined actions of these three motors.

Identification and partial characterization of mitotic centromere-associated kinesin, a kinesin-related protein that associates with centromeres during mitosis

Using antipeptide antibodies to conserved regions of the kinesin motor domain, we cloned a kinesin-related protein that associates with the centromere region of mitotic chromosomes. We call the protein MCAK, for mitotic centromere-associated kinesin. MCAK appears concentrated on centromeres at prophase and persists until telophase, after which time the localization disperses. It is found throughout the centromere region and between the kinetochore plates of isolated mitotic CHO chromosomes, in contrast to two other kinetochore-associated microtubule motors: cytoplasmic dynein and CENP-E (Yen et al., 1992), which are closer to the outer surface of the kinetochore plates. Sequence analysis shows MCAK to be a kinesin-related protein with the motor domain located in the center of the protein. It is 60-70% similar to kif2, a kinesin-related protein originally cloned from mouse brain with a centrally located motor domain (Aizawa et al., 1992). MCAK protein is present in interphase and mitotic CHO cells and is transcribed as a single 3.4-kb message.

The formation and functioning of yeast mitotic spindles

The mitotic spindle contains the machinery responsible for sister chromatid segregation. It is composed of a complex and dynamic array of microtubules, which are nucleated from the spindle poles. Studies of yeast spindle functions by molecular genetic analysis and by in vitro functional analysis have identified proteins that are mitosis-specific and present at very low concentrations in the cell, and have revealed the molecular bases of several processes required for the formation and functioning of the mitotic spindle. Here I review the current knowledge of the processes that are common to most eukaryotes: microtubule nucleation at the spindle poles, bipolar spindle assembly, maintenance of the spindle structure, chromosome attachment to the spindle and chromosome separation on the spindle.

KIF1B, a novel microtubule plus end-directed monomeric motor protein for transport of mitochondria

To further elucidate the mechanism of organelle transport, we cloned a novel member of the mouse kinesin superfamily, KIF1B. This N-terminal-type motor protein is expressed ubiquitously in various kinds of tissues. In situ hybridization revealed that KIF1B is expressed abundantly in differentiated nerve cells. Interestingly, K1F1B works as a monomer, having a microtubule plus end-directed motility. Our rotary shadowing electron microscopy revealed mostly single globular structures. Immunocytochemically, KIF1B was colocalized with mitochondria in vivo. Furthermore, a subcellular fractionation study showed that KIF1B was concentrated in the mitochondrial fraction, and purified K1F1B could transport mitochondria along microtubules in vitro. These data strongly suggested that KIF1B works as a monomeric motor for anterograde transport of mitochondria.

STU1, a suppressor of a beta-tubulin mutation, encodes a novel and essential component of the yeast mitotic spindle

We have isolated a cold-sensitive allele of TUB2, the sole gene encoding beta-tubulin in S. cerevisiae, that confers a specific defect in spindle microtubule function. At 14 degrees C, tub2-406 cells lack a normal bipolar spindle but do assemble functional cytoplasmic microtubules. In an attempt to identify proteins that are important for spindle assembly, we screened for suppressors of the cold-sensitivity of tub2-406 and obtained four alleles of a novel gene, STU1. Genetic interactions between stu1 alleles and alleles of TUB1 and TUB2 suggest that Stu1p specifically interacts with microtubules. STU1 is essential for growth and disruption of STU1 causes defects in spindle assembly that are similar to those produced by the tub2-406 mutation. The nucleotide sequence of the STU1 gene predicts a protein product of 174 kD with no significant similarity to known proteins. An epitope-tagged Stulp colocalizes with microtubules in the mitotic spindle of yeast. These results demonstrate that Stulp is an essential component of the yeast mitotic spindle.

Heterogeneity and microtubule interaction of the CHO1 antigen, a mitosis-specific kinesin-like protein. Analysis of subdomains expressed in insect sf9 cells

The CHO1 antigen is a mitosis-specific kinesin-like motor located at the interzonal region of the spindle. The human cDNA coding for the antigen contains a domain with sequence similarity to the motor domain of kinesin-like protein (Nislow et al., Nature 359, 543, 1992). Here we cloned cDNAs encoding the CHO1 antigen by immunoscreening of a CHO Uni-Zap expression library, the same species in which the original monoclonal antibody was raised. cDNAs of CHO cells encode a 953 amino acid polypeptide with a calculated molecular mass of 109 kDa. The N-terminal 73% of the antigen was 87% identical to the human clone, whereas the remaining 27% of the coding region showed only 48% homology. Insect Sf9 cells infected with baculovirus containing the full-length insert produced 105 and 95 kDa polypeptides, the same doublet identified as the original antigen in CHO cells. Truncated polypeptides corresponding to the N-terminal motor and C-terminal tail produced a 56 and 54 kDa polypeptide in Sf9 cells, respectively. Full and N-terminal proteins co-sedimented with, and caused bundling of, brain microtubules in vitro, whereas the C-terminal polypeptide did not. Cells expressing the N terminus formed one or more cytoplasmic processes. Immunofluorescence as well as electron microscopic observations revealed the presence of thick bundles of microtubules, which were closely packed, forming a marginal ring just beneath the cell membrane and a core in the processes. The diffusion coefficient and sedimentation coefficient were determined for the native CHO1 antigen by gel filtration and sucrose density gradient centrifugation, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)

Factors required for the binding of reassembled yeast kinetochores to microtubules in vitro

Kinetochores are structures that assemble on centromeric DNA and mediate the attachment of chromosomes to the microtubules of the mitotic spindle. The protein components of kinetochores are poorly understood, but the simplicity of the S. cerevisiae kinetochore makes it an attractive candidate for molecular dissection. Mutations in genes encoding CBF1 and CBF3, proteins that bind to yeast centromeres, interfere with chromosome segregation in vivo. To determine the roles played by these factors and by various regions of centromeric DNA in kinetochore function, we have developed a method to partially reassemble kinetochores on exogenous centromeric templates in vitro and to visualize the attachment of these reassembled kinetochore complexes to microtubules. In this assay, single reassembled complexes appear to mediate microtubule binding. We find that CBF3 is absolutely essential for this attachment but, contrary to previous reports (Hyman, A. A., K. Middleton, M. Centola, T.J. Mitchison, and J. Carbon. 1992. Microtubule-motor activity of a yeast centromere-binding protein complex. Nature (Lond.). 359:533-536) is not sufficient. Additional cellular factors interact with CBF3 to form active microtubule-binding complexes. This is mediated primarily by the CDEIII region of centromeric DNA but CDEII plays an essential modulatory role. Thus, the attachment of kinetochores to microtubules appears to involve a hierarchy of interactions by factors that assemble on a core complex consisting of DNA-bound CBF3.

The spindle pole body of yeast

Microtubule organizing centers play an essential cellular role in nucleating microtubule assembly and establishing the microtubule array. The microtubule organizing center of yeast, the spindle pole body (SPB), shares many functions and properties with those other organisms. In recent years considerable new information has been generated concerning components associated with the SPB, and the mechanism by which it duplicates. This article reviews our current view of the cytology and molecular composition of the SPB of the budding yeast, Saccharomyces cerevisiae, and the fission yeast, Schizosaccharomyces pombe. Genetic studies in these organisms has revealed information about how the SPB duplicates and separates, and its roles during vegetative growth, mating and meiosis.

Fission yeast minichromosome loss mutants mis cause lethal aneuploidy and replication abnormality

Precise chromosome transmission in cell division cycle is maintained by a number of genes. The attempt made in the present study was to isolate temperature-sensitive (ts) fission yeast mutants that display high loss rates of minichromosomes at permissive or semipermissive temperature (designated mis). By colony color assay of 539 ts strains that contain a minichromosome, we have identified 12 genetic loci (mis1-mis12) and determined their phenotypes at restrictive temperature. Seven of them are related to cell cycle block phenotype at restrictive temperature, three of them in mitosis. Unequal distribution of regular chromosomes in the daughters is extensive in mis6 and mis12. Cells become inviable after rounds of cell division due to missegregation. The phenotype of mis5 is DNA replication defect and hypersensitivity to UV ray and hydroxyurea. mis5+ encodes a novel member of the ubiquitous MCM family required for the onset of replication. The mis5+ gene is essential for viability and functionally distinct from other previously identified members in fission yeast, cdc21+, nda1+, and nda4+. The mis11 mutant phenotype was the cell division block with reduced cell size. Progression of the G1 and G2 phases is blocked in mis11. The cloned mis11+ gene is identical to prp2+, which is essential for RNA splicing and similar to a mammalian splicing factor U2AF65.

The essential yeast Tcp1 protein affects actin and microtubules

Previously, we showed that the yeast Saccharomyces cerevisiae cold-sensitive mutation tcp1-1 confers growth arrest concomitant with cytoskeletal disorganization and disruption of microtubule-mediated processes. We have identified two new recessive mutations, tcp1-2 and tcp1-3, that confer heat- and cold-sensitive growth. Cells carrying tcp1 alleles were analyzed after exposure to the appropriate restrictive temperatures by cell viability tests, differential contrast microscopy, fluorescent, and immunofluorescent microscopy of DNA, tubulin, and actin and by determining the DNA content per cell. All three mutations conferred unique phenotypes indicative of cytoskeletal dysfunction. A causal relationship between loss of Tcp1p function and the development of cytoskeletal abnormalities was established by double mutant analyses. Novel phenotypes indicative of allele-specific genetic interactions were observed when tcp1-1 was combined in the same strain with tub1-1, tub2-402, act1-1, and act1-4, but not with other tubulin or actin mutations or with mutations in other genes affecting the cytoskeleton. Also, overproduction of wild-type Tcp1p partially suppressed growth defects conferred by act1-1 and act1-4. Furthermore, Tcp1p was localized to the cytoplasm and the cell cortex. Based on our results, we propose that Tcp1p is required for normal development and function of actin and microtubules either through direct or indirect interaction with the major cytoskeletal components.

DBF8, an essential gene required for efficient chromosome segregation in Saccharomyces cerevisiae

To investigate chromosome segregation in Saccharomyces cerevisiae, we examined a collection of temperature-sensitive mutants that arrest as large-budded cells at restrictive temperatures (L. H. Johnston and A. P. Thomas, Mol. Gen. Genet. 186:439-444, 1982). We characterized dbf8, a mutation that causes cells to arrest with a 2c DNA content and a short spindle. DBF8 maps to chromosome IX near the centromere, and it encodes a 36-kDa protein that is essential for viability at all temperatures. Mutational analysis reveals that three dbf8 alleles are nonsense mutations affecting the carboxy-terminal third of the encoded protein. Since all of these mutations confer temperature sensitivity, it appears that the carboxyl-terminal third of the protein is essential only at a restrictive temperature. In support of this conclusion, an insertion of URA3 at the same position also confers a temperature-sensitive phenotype. Although they show no evidence of DNA damage, dbf8 mutants exhibit increased rates of chromosome loss and nondisjunction even at a permissive temperature. Taken together, our data suggest that Dbf8p plays an essential role in chromosome segregation.

A "slow" homotetrameric kinesin-related motor protein purified from Drosophila embryos

Pan-kinesin peptide antibodies (Cole, D. G., Cande, W. Z., Baskin, R. J., Skoufias, D. A., Hogan, C. J., and Scholey, J. M. (1992) J. Cell Sci. 101, 291-301; Sawin, K. E., Mitchinson, T. J., and Wordeman, L. G. (1992) J. Cell Sci. 101, 303-313) were used to identify and isolate kinesin-related proteins (KRPs) from Drosophila melanogaster embryonic cytosol. These KRPs cosedimented with microtubules (MTs) polymerized from cytosol treated with AMP-PNP (adenyl-5'-yl imidodiphosphate), and one of them, KRP130, was further purified from ATP eluates of the embryonic MTs. Purified KRP130 behaves as a homotetrameric complex composed of four 130-kDa polypeptide subunits which displays a "slow" plus-end directed motor activity capable of moving single MTs at 0.04 +/- 0.01 microns/s. The 130-kDa subunit of KRP130 was tested for reactivity with monoclonal and polyclonal antibodies that are specific for various members of the kinesin superfamily. Results indicate that the KRP130 subunit is related to Xenopus Eg5 (Sawin, K. E., Le Guellec, K. L., Philippe, M., Mitchinson, T. J. (1992) Nature 359, 540-543), a member of the BimC subfamily of kinesins. Therefore, KRP130 appears to be the first Drosophila KRP, and the first member of the BimC subfamily in any organism, to be purified from native tissue as a multimeric motor complex.

DBF8, an essential gene required for efficient chromosome segregation in Saccharomyces cerevisiae

To investigate chromosome segregation in Saccharomyces cerevisiae, we examined a collection of temperature-sensitive mutants that arrest as large-budded cells at restrictive temperatures (L. H. Johnston and A. P. Thomas, Mol. Gen. Genet. 186:439-444, 1982). We characterized dbf8, a mutation that causes cells to arrest with a 2c DNA content and a short spindle. DBF8 maps to chromosome IX near the centromere, and it encodes a 36-kDa protein that is essential for viability at all temperatures. Mutational analysis reveals that three dbf8 alleles are nonsense mutations affecting the carboxy-terminal third of the encoded protein. Since all of these mutations confer temperature sensitivity, it appears that the carboxyl-terminal third of the protein is essential only at a restrictive temperature. In support of this conclusion, an insertion of URA3 at the same position also confers a temperature-sensitive phenotype. Although they show no evidence of DNA damage, dbf8 mutants exhibit increased rates of chromosome loss and nondisjunction even at a permissive temperature. Taken together, our data suggest that Dbf8p plays an essential role in chromosome segregation.

Sequencing and characterization of the kinesin-related genes katB and katC of Arabidopsis thaliana

Complementary DNAs of two kinesin-related genes, katB and katC, were isolated from Arabidopsis thaliana and sequenced. The carboxyl-terminal regions of the polypeptides encoded by these genes, especially the presumptive ATP-binding and microtubule-binding domains, share significant sequence homology with the mechanochemical motor domain of the kinesin heavy chain. The predicted secondary structures of KatB and KatC proteins include a large globular domain in the carboxyl-terminal region and a small globular domain in the amino-terminal region that are separated by a long alpha-helical coiled-coil with heptad repeats. A truncated KatC polypeptide (KatC(207-754)), which includes the carboxyl-terminal region of KatC, was expressed in Escherichia coli and was shown to possess microtubule-stimulated ATPase activity and to bind to microtubules in an ATP-sensitive manner, both of which are characteristics of kinesin and kinesin-like proteins.

Localization of the mei-1 gene product of Caenorhaditis elegans, a meiotic-specific spindle component

Genetic evidence suggests that the product of the mei-1 gene of Caenorhabditis elegans is specifically required for meiosis in the female germline. Loss-of-function mei-1 mutations block meiotic spindle formation while a gain-of-function allele instead results in spindle defects during the early mitotic cleavages. In this report, we use immunocytochemistry to examine the localization of the mei-1 product in wild-type and mutant embryos. During metaphase of meiosis I in wild-type embryos, mei-1 protein was found throughout the spindle but was more concentrated toward the poles. At telophase I, mei-1 product colocalized with the chromatin at the spindle poles. The pattern was repeated during meiosis II but no mei-1 product was visible during the subsequent mitotic cleavages. The mei-1 gain-of-function allele resulted in ectopic mei-1 staining in the centers of the microtubule-organizing centers during interphase and in the spindles during the early cleavages. This aberrant localization is probably responsible for the poorly formed and misoriented cleavage spindles characteristic of the mutation. We also examined the localization of mei-1(+) product in the presence of mutations of genes that genetically interact with mei-1 alleles. mei-2 is apparently required to localize mei-1 product to the spindle during meiosis while mel-26 acts as a postmeiotic inhibitor. We conclude that mei-1 encodes a novel spindle component, one that is specialized for the acentriolar meiotic spindles unique to female meiosis. The genes mei-2 and mel-26 are part of a regulatory network that confines mei-1 activity to meiosis.

The Chlamydomonas FLA10 gene encodes a novel kinesin-homologous protein

Many genes on the uni linkage group of Chlamydomonas affect the basal body/flagellar apparatus. Among these are five FLA genes, whose mutations cause temperature-sensitive defects in flagellar assembly. We present the molecular analysis of a gene which maps to fla10 and functionally rescues the fla10 phenotype. Nucleotide sequencing revealed that the gene encodes a kinesin-homologous protein, KHP1. The 87-kD predicted KHP1 protein, like kinesin heavy chain, has an amino-terminal motor domain, a central alpha-helical stalk, and a basic, globular carboxy-terminal tail. Comparison to other kinesin superfamily members indicated striking similarity (64% identity in motor domains) to a mouse gene, KIF3, expressed primarily in cerebellum. In synchronized cultures, the KHP1 mRNA accumulated after cell division, as did flagellar dynein mRNAs. KHP1 mRNA levels also increased following deflagellation. Polyclonal antibodies detected KHP1 protein in Western blots of purified flagella and axonemes. The protein was partially released from axonemes with ATP treatment, but not with AMP-PNP. Western blot analysis of axonemes from various motility mutants suggested that KHP1 is not a component of radial spokes, dynein arms, or the central pair complex. The quantity of KHP1 protein in axonemes of the mutant fla10-1 was markedly reduced, although no reduction was observed in two other uni linkage group mutants, fla9 and fla11. Furthermore, fla10-1 was rescued by transformation with KHP1 genomic DNA. These results indicate that KHP1 is the gene product of FLA10 and suggest a novel role for this kinesin-related protein in flagellar assembly and maintenance.

Molecular phylogeny of the kinesin family of microtubule motor proteins

The rapidly expanding kinesin family of microtubule motor proteins includes proteins that are involved in diverse microtubule-based functions in the cell. Phylogenetic analysis of the motor regions of the kinesin proteins reveals at least five clearly defined groups that are likely to identify kinesins with different roles in basic cellular processes. Two of the groups are consistent with overall sequence similarity, while two groups contain proteins that are related in overall structure or function but show no significant sequence similarity outside the motor domain. One of these groups consists only of kinesin proteins with predicted C-terminal motor domains; another includes only kinesins required for mitotic spindle bipolarity. Drosophila Nod, presently an ungrouped protein, may represent a class of kinesins that, like the myosin I proteins, function as monomers. The analysis indicates that many types of kinesin proteins exist in eukaryotic organisms. At least two of the five groups identified in this analysis are expected to be present in most, or all, eukaryotes.

A new kinesin-like protein (Klp1) localized to a single microtubule of the Chlamydomonas flagellum

The kinesin superfamily of mechanochemical proteins has been implicated in a wide variety of cellular processes. We have begun studies of kinesins in the unicellular biflagellate alga, Chlamydomonas reinhardtii. A full-length cDNA, KLP1, has been cloned and sequenced, and found to encode a new member of the kinesin superfamily. An antibody was raised against the nonconserved tail region of the Klp1 protein, and it was used to probe for Klp1 in extracts of isolated flagella and in situ. Immunofluorescence of whole cells indicated that Klp1 was present in both the flagella and cell bodies. In wild-type flagella, Klp1 was found tightly to the axoneme; immunogold labeling of wild-type axonemal whole mounts showed that Klp1 was restricted to one of the two central pair microtubules at the core of the axoneme. Klp1 was absent from the flagella of mutants lacking the central pair microtubules, but was present in mutant flagella from pf16 cells, which contain an unstable C1 microtubule, indicating that Klp1 was bound to the C2 central pair microtubule. Localization of Klp1 to the C2 microtubule was confirmed by immunogold labeling of negatively stained and thin-sectioned axonemes. These findings suggest that Klp1 may play a role in rotation or twisting of the central pair microtubules.