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The yeast motor protein, Kar3p, is essential for meiosis I.

Bascom-Slack CA, Dawson DS - J. Cell Biol. (1997)

Bottom Line: The recognition and alignment of homologous chromosomes early in meiosis is essential for their subsequent segregation at anaphase I; however, the mechanism by which this occurs is unknown.We demonstrate here that, in the absence of the molecular motor, Kar3p, meiotic cells are blocked with prophase monopolar microtubule arrays and incomplete synaptonemal complex (SC) formation. kar3 mutants exhibit very low levels of heteroallelic recombination. kar3 mutants do produce double-strand breaks that act as initiation sites for meiotic recombination in yeast, but at levels severalfold reduced from wild-type.These data are consistent with a meiotic role for Kar3p in the events that culminate in synapsis of homologues.

View Article: PubMed Central - PubMed

Affiliation: Department of Molecular Biology and Microbiology, Tufts University School of Medicine, Boston, Massachusetts 02111, USA.

ABSTRACT
The recognition and alignment of homologous chromosomes early in meiosis is essential for their subsequent segregation at anaphase I; however, the mechanism by which this occurs is unknown. We demonstrate here that, in the absence of the molecular motor, Kar3p, meiotic cells are blocked with prophase monopolar microtubule arrays and incomplete synaptonemal complex (SC) formation. kar3 mutants exhibit very low levels of heteroallelic recombination. kar3 mutants do produce double-strand breaks that act as initiation sites for meiotic recombination in yeast, but at levels severalfold reduced from wild-type. These data are consistent with a meiotic role for Kar3p in the events that culminate in synapsis of homologues.

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Zip1 staining pattern of kar3 cells. Aliquots of  isogenic KAR3 (Dd519 +  MR820) and kar3 (Dd519)  meiotic cultures were harvested at pachytene, 14 h after induction of sporulation.  Nuclei were spread and  stained with anti-Zip1 and  the DNA-specific dye, DAPI.  (a) The left panel shows the  Zip1 staining pattern of a  typical wild-type pachytene  nucleus; the corresponding  staining with DAPI is shown  on the right. (b and c) The  left-hand panels are Zip1-stained nuclei representative  of those with dots or elongated signals. The corresponding DAPI images are shown on the right. Brightly stained foci indicative of polycomplexes were observed in 25% of the kar3  cells, and 2.5% of the wild-type cells (arrows). Bar, 1 μm.
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Figure 6: Zip1 staining pattern of kar3 cells. Aliquots of isogenic KAR3 (Dd519 + MR820) and kar3 (Dd519) meiotic cultures were harvested at pachytene, 14 h after induction of sporulation. Nuclei were spread and stained with anti-Zip1 and the DNA-specific dye, DAPI. (a) The left panel shows the Zip1 staining pattern of a typical wild-type pachytene nucleus; the corresponding staining with DAPI is shown on the right. (b and c) The left-hand panels are Zip1-stained nuclei representative of those with dots or elongated signals. The corresponding DAPI images are shown on the right. Brightly stained foci indicative of polycomplexes were observed in 25% of the kar3 cells, and 2.5% of the wild-type cells (arrows). Bar, 1 μm.

Mentions: Many mutations that prevent interhomologue recombination also prevent complete SC formation. To test the ability of kar3 mutants to form SC, we immunostained spread meiotic chromosomes harvested 14 h after induction of meiosis with antibody to Zip1, a component of the SC (Sym et al., 1993). Approximately equal fractions of mutant and wild-type nuclei showed Zip1 staining, indicating that the kar3 mutation does not block progression to this stage. In wild-type samples, most Zip1-stained nuclei (79%, n = 160) exhibited worm-like structures indicative of full-length SC. In many of these nuclei ∼16 synapsed pairs were clearly evident (Fig. 6 a). Mutant nuclei exhibited a range of Zip1 staining patterns, though none showed characteristic full-length SC staining. 46% (n = 129) of the kar3 Zip1-stained nuclei showed numerous dots or elongated signals indicative of partial or discontinuous SC formation (Fig. 6, b and c; the remaining 56% showed largely homogeneous nuclear staining). ∼25% of the kar3 cells (32 of 129 spreads) with Zip1 staining contained a single brightly stained structure (Fig. 6, b and c), which probably indicates the presence of a polycomplex. These structures were seen in <3% of the wild-type spreads (4 of 160) at the same time point. Polycomplexes are common in pachytene nuclei of mutants unable to assemble SC (Alani et al., 1990; Bishop et al., 1992; Loidl et al., 1994), or can result from overexpression of Zip1p (Sym and Roeder, 1995). The SC defect of the kar3 mutants does not improve over time. At a later time point, when >50% of the KAR3 cells have produced tetrads, the DNA in the kar3 mutants has decondensed and the Zip1 staining is barely visible (data not shown).


The yeast motor protein, Kar3p, is essential for meiosis I.

Bascom-Slack CA, Dawson DS - J. Cell Biol. (1997)

Zip1 staining pattern of kar3 cells. Aliquots of  isogenic KAR3 (Dd519 +  MR820) and kar3 (Dd519)  meiotic cultures were harvested at pachytene, 14 h after induction of sporulation.  Nuclei were spread and  stained with anti-Zip1 and  the DNA-specific dye, DAPI.  (a) The left panel shows the  Zip1 staining pattern of a  typical wild-type pachytene  nucleus; the corresponding  staining with DAPI is shown  on the right. (b and c) The  left-hand panels are Zip1-stained nuclei representative  of those with dots or elongated signals. The corresponding DAPI images are shown on the right. Brightly stained foci indicative of polycomplexes were observed in 25% of the kar3  cells, and 2.5% of the wild-type cells (arrows). Bar, 1 μm.
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Figure 6: Zip1 staining pattern of kar3 cells. Aliquots of isogenic KAR3 (Dd519 + MR820) and kar3 (Dd519) meiotic cultures were harvested at pachytene, 14 h after induction of sporulation. Nuclei were spread and stained with anti-Zip1 and the DNA-specific dye, DAPI. (a) The left panel shows the Zip1 staining pattern of a typical wild-type pachytene nucleus; the corresponding staining with DAPI is shown on the right. (b and c) The left-hand panels are Zip1-stained nuclei representative of those with dots or elongated signals. The corresponding DAPI images are shown on the right. Brightly stained foci indicative of polycomplexes were observed in 25% of the kar3 cells, and 2.5% of the wild-type cells (arrows). Bar, 1 μm.
Mentions: Many mutations that prevent interhomologue recombination also prevent complete SC formation. To test the ability of kar3 mutants to form SC, we immunostained spread meiotic chromosomes harvested 14 h after induction of meiosis with antibody to Zip1, a component of the SC (Sym et al., 1993). Approximately equal fractions of mutant and wild-type nuclei showed Zip1 staining, indicating that the kar3 mutation does not block progression to this stage. In wild-type samples, most Zip1-stained nuclei (79%, n = 160) exhibited worm-like structures indicative of full-length SC. In many of these nuclei ∼16 synapsed pairs were clearly evident (Fig. 6 a). Mutant nuclei exhibited a range of Zip1 staining patterns, though none showed characteristic full-length SC staining. 46% (n = 129) of the kar3 Zip1-stained nuclei showed numerous dots or elongated signals indicative of partial or discontinuous SC formation (Fig. 6, b and c; the remaining 56% showed largely homogeneous nuclear staining). ∼25% of the kar3 cells (32 of 129 spreads) with Zip1 staining contained a single brightly stained structure (Fig. 6, b and c), which probably indicates the presence of a polycomplex. These structures were seen in <3% of the wild-type spreads (4 of 160) at the same time point. Polycomplexes are common in pachytene nuclei of mutants unable to assemble SC (Alani et al., 1990; Bishop et al., 1992; Loidl et al., 1994), or can result from overexpression of Zip1p (Sym and Roeder, 1995). The SC defect of the kar3 mutants does not improve over time. At a later time point, when >50% of the KAR3 cells have produced tetrads, the DNA in the kar3 mutants has decondensed and the Zip1 staining is barely visible (data not shown).

Bottom Line: The recognition and alignment of homologous chromosomes early in meiosis is essential for their subsequent segregation at anaphase I; however, the mechanism by which this occurs is unknown.We demonstrate here that, in the absence of the molecular motor, Kar3p, meiotic cells are blocked with prophase monopolar microtubule arrays and incomplete synaptonemal complex (SC) formation. kar3 mutants exhibit very low levels of heteroallelic recombination. kar3 mutants do produce double-strand breaks that act as initiation sites for meiotic recombination in yeast, but at levels severalfold reduced from wild-type.These data are consistent with a meiotic role for Kar3p in the events that culminate in synapsis of homologues.

View Article: PubMed Central - PubMed

Affiliation: Department of Molecular Biology and Microbiology, Tufts University School of Medicine, Boston, Massachusetts 02111, USA.

ABSTRACT
The recognition and alignment of homologous chromosomes early in meiosis is essential for their subsequent segregation at anaphase I; however, the mechanism by which this occurs is unknown. We demonstrate here that, in the absence of the molecular motor, Kar3p, meiotic cells are blocked with prophase monopolar microtubule arrays and incomplete synaptonemal complex (SC) formation. kar3 mutants exhibit very low levels of heteroallelic recombination. kar3 mutants do produce double-strand breaks that act as initiation sites for meiotic recombination in yeast, but at levels severalfold reduced from wild-type. These data are consistent with a meiotic role for Kar3p in the events that culminate in synapsis of homologues.

Show MeSH
Related in: MedlinePlus