A number of lines of evidence point to a predominance of cytokinesis defects in spermatogenesis in hypomorphic alleles of the Drosophila polo gene. In the pre-meiotic mitoses, cytokinesis defects result in cysts of primary spermatocytes with reduced numbers of cells that can contain multiple centrosomes. These are connected by a correspondingly reduced number of ring canals, structures formed by the stabilization of the cleavage furrow. The earliest defects during the meiotic divisions are a failure to form the correct mid-zone and mid-body structures at telophase. This is accompanied by a failure to correctly localize the Pavarotti kinesin- like protein that functions in cytokinesis, and of the septin Peanut and of actin to be incorporated into a contractile ring. In spite of these defects, cyclin B is degraded and the cells exit M phase. The resulting spermatids are frequently binuclear or tetranuclear, in which case they develop either two or four axonemes, respectively. A significant proportion of spermatids in which cytokinesis has failed may also show the segregation defects previously ascribed to polo1 mutants. We discuss these findings in respect to conserved functions for the Polo-like kinases in regulating progression through M phase, including the earliest events of cytokinesis.

Pav-KLP is the Drosophila member of the MKLP1 family essential for cytokinesis. In the syncytial blastoderm embryo, GFP-Pav-KLP cyclically associates with astral, spindle, and midzone microtubules and also to actomyosin pseudocleavage furrows. As the embryo cellularizes, GFP-Pav-KLP also localizes to the leading edge of the furrows that form cells. In mononucleate cells, nuclear localization of GFP-Pav-KLP is mediated through NLS elements in its C-terminal domain. Mutants in these elements that delocalize Pav-KLP to the cytoplasm in interphase do not affect cell division. In mitotic cells, one population of wild-type GFP-Pav-KLP associates with the spindle and concentrates in the midzone at anaphase B. A second is at the cell cortex on mitotic entry and later concentrates in the region of the cleavage furrow. An ATP binding mutant does not localize to the cortex and spindle midzone but accumulates on spindle pole microtubules to which actin is recruited. This leads either to failure of the cleavage furrow to form or later defects in which daughter cells remain connected by a microtubule bridge. Together, this suggests Pav-KLP transports elements of the actomyosin cytoskeleton to plus ends of astral microtubules in the equatorial region of the cell to permit cleavage ring formation.

A cDNA from a novel Ca 2؉ -dependent member of the mitochondrial solute carrier superfamily was isolated from a rabbit small intestinal cDNA library. The full-length cDNA clone was 3,298 nt long and coded for a protein of 475 amino acids, with four elongation factor-hand motifs located in the N-terminal half of the molecule. The 25-kDa N-terminal polypeptide was expressed in Escherichia coli, and it was demonstrated that it bound Ca 2؉ , undergoing a reversible and specific conformational change as a result. The conformation of the polypeptide was sensitive to Ca 2؉ which was bound with high affinity (K d Ϸ 0.37 M), the apparent Hill coefficient for Ca 2؉ -induced changes being about 2.0. The deduced amino acid sequence of the C-terminal half of the molecule revealed 78% homology to Grave disease carrier protein and 67% homology to human ADP͞ATP translocase; this sequence homology identified the protein as a new member of the mitochondrial transporter superfamily. Northern blot analysis revealed the presence of a single transcript of about 3,500 bases, and low expression of the transporter could be detected in the kidney but none in the liver. The main site of expression was the colon with smaller amounts found in the small intestine proximal to the ileum. Immunoelectron microscopy localized the transporter in the peroxisome, although a minor fraction was found in the mitochondria. The Ca 2؉ binding N-terminal half of the transporter faces the cytosol.Peroxisomes and mitochondria are the two organelles present in mammalian eukaryotic cells for which an evolutionary endosymbiotic origin has been proposed (1, 2). Both organelles share a set of similar metabolic pathways, are the site of cellular oxygen consumption, and have similar macromolecular components and membrane phospholipid compositions (3). Mitochondria, in contrast to peroxisomes contain their own DNA and are surrounded by a double membrane rather than a single one. Despite the existence of mitochondrial DNA, most proteins are coded for by nuclear genes, synthesized on free cytosolic polysomes and subsequently posttranslationally sorted to their final mitochondrial location (for a review, see ref. 4). In principle, the same applies for peroxisomal proteins although much less is known about protein import in this organelle (for a review, see ref. 5).The mitochondrial inner membrane contains three different major classes of proteins: the electron transport chain complex, the ATP synthase complex, and the mitochondrial solute carriers. The first two complexes originated at the prokariotic level whereas the mitochondrial solute carriers must have developed when the ancestor prokaryote became symbiotic in the eukaryotic cell as they fulfill a new demand of the eukaryote for intensive traffic of metabolites between the cytosolic and matrix space (6