Synaptonemal complex (SC) component Zip1 plays a role in meiotic recombination independent of SC polymerization along the chromosomes.

Zip1 is a yeast synaptonemal complex (SC) central region component and is required for normal meiotic recombination and crossover interference. Physical analysis of meiotic recombination in a zip1 mutant reveals the following: Crossovers appear later than normal and at a reduced level. Noncrossover recombinants, in contrast, seem to appear in two phases: (i) a normal number appear with normal timing and (ii) then additional products appear late, at the same time as crossovers. Also, Holliday junctions are present at unusually late times, presumably as precursors to late-appearing products. Red1 is an axial structure component required for formation of cytologically discernible axial elements and SC and maximal levels of recombination. In a red1 mutant, crossovers and noncrossovers occur at coordinately reduced levels but with normal timing. If Zip1 affected recombination exclusively via SC polymerization, a zip1 mutation should confer no recombination defect in a red1 strain background. But a red1 zip1 double mutant exhibits the sum of the two single mutant phenotypes, including the specific deficit of crossovers seen in a zip1 strain. We infer that Zip1 plays at least one role in recombination that does not involve SC polymerization along the chromosomes. Perhaps some Zip1 molecules act first in or around the sites of recombinational interactions to influence the recombination process and thence nucleate SC formation. We propose that a Zip1-dependent, pre-SC transition early in the recombination reaction is an essential component of meiotic crossover control. A molecular basis for crossover/noncrossover differentiation is also suggested.

Yeast actin: polymerization kinetic studies of wild type and a poorly polymerizing mutant.

Wild-type actin and a mutant actin were isolated from yeast (Saccharomyces cerevisiae) and the polymerization properties were examined at pH 8.0 and 20 degrees C. The polymerization reaction was followed either by an increase in pyrene-labeled actin fluorescence or by a decrease in intrinsic fluorescence in the absence of pyrene-labeled actin. While similar to the properties of skeletal muscle actin, there are several important differences between the wild-type yeast and muscle actins. First, yeast actin polymerizes more rapidly than muscle actin under the same experimental conditions. The difference in rates may result from a difference in the steps involving formation of the nucleating species. Second, as measured with pyrene-labeled yeast actin, but not with intrinsic fluorescence, there is an overshoot in the fluorescence that has not been observed with skeletal muscle actin under the same conditions. Third, in order to simulate the polymerization process of wild-type yeast actin it is necessary to assume some fragmentation of the filaments. Finally, gelsolin inhibits polymerization of yeast actin but is known to accelerate the polymerization of muscle actin. A mutant actin (R177A/D179A) has also been isolated and studied. The mutations are at a region of contact between monomers across the long axis of the actin filament. This mutant polymerizes more slowly than wild type and filaments do not appear to fragment during polymerization. Elongation rates of the wild type and the mutant differ by only about 3-fold, and the slower polymerization of the mutant appears to result primarily from poorer nucleation.

Roles of heavy and light chains in IgM polymerization.

IgM antibodies are secreted as multisubunit polymers that consist of as many as three discrete polypeptides: mu heavy chains, light (L) chains, and joining (J) chains. We wished to determine whether L chains that are required to confer secretory competence on immunoglobulin molecules must be present for IgM to polymerize--that is, for intersubunit disulfide bonds to form between mu chains. Using a L-chain-loss variant of an IgM-secreting hybridoma, we demonstrated that mu chains were efficiently polymerized independent of L chains, in a manner similar to that observed for conventional microL complexes, and that the mu polymers incorporated J chain. These mu polymers were not secreted but remained associated with the endoplasmic reticulum-resident chaperone BiP (GRP78). This finding is consistent with the endoplasmic reticulum being the subcellular site of IgM polymerization. We conclude that mu chain alone has the potential to direct the polymerization of secreted IgM, a process necessary but not sufficient for IgM to attain secretory competence.

Radial and tangential dispersion patterns in the mouse retina are cell-class specific.

The retina is derived from a pseudostratified germinal zone in which the relative position of a progenitor cell is believed to determine the position of the progeny aligned in the radial axis. Such a developmental mechanism would ensure that radial arrays of cells which comprise functional units in the mature central nervous system are also clonally related. The present study has tested this hypothesis by using X chromosome-inactivation transgenic mosaic mice. We report that the retina shows a conspicuous distinction for clonally related neuroblasts of different laminar and functional fates: the rod photoreceptor, Müller, and bipolar cells are aligned in the radial axis, whereas the cone photoreceptor, horizontal, amacrine, and ganglion cells are tangentially displaced with respect to them. These results indicate that the dispersion of cell classes across the retinal surface is differentially constrained. Some classes of retinal neuroblast exhibit a significant tangential, as well as radial, component in their dispersion from the germinal zone, whereas others disperse only in the radial dimension. Consequently, the majority of radial columns within the mature retina must be derived from multiple progenitors. Because the cone photoreceptor, horizontal, amacrine, and ganglion cells establish nonrandom matrices in their cellular distributions within the respective retinal layers, tangential dispersion may be the means by which these matrices are constructed.