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Department of Biology, University of California, San Diego, La Jolla, California 92093
Nuclear and mitochondrial transmission to
daughter buds of Saccharomyces cerevisiae depends on
Mdm1p, an intermediate filament-like protein localized
to numerous punctate structures distributed throughout the yeast cell cytoplasm. These structures disappear and organelle inheritance is disrupted when mdm1 mutant cells are incubated at the restrictive temperature.
To characterize further the function of Mdm1p, new
mutant mdm1 alleles that confer temperature-sensitive
growth and defects in organelle inheritance but produce stable Mdm1p structures were isolated. Microscopic analysis of the new mdm1 mutants revealed
three phenotypic classes: Class I mutants showed defects in both mitochondrial and nuclear transmission;
Class II alleles displayed defective mitochondrial inheritance but had no effect on nuclear movement; and
Class III mutants showed aberrant nuclear inheritance
but normal mitochondrial distribution. Class I and II
mutants also exhibited altered mitochondrial morphology, possessing primarily small, round mitochondria
instead of the extended tubular structures found in
wild-type cells. Mutant mdm1 alleles affecting nuclear
transmission were of two types: Class Ia and IIIa mutants were deficient for nuclear movement into daughter buds, while Class Ib and IIIb mutants displayed a
complete transfer of all nuclear DNA into buds. The
mutations defining all three allelic classes mapped to
two distinct domains within the Mdm1p protein. Genetic crosses of yeast strains containing different mdm1
alleles revealed complex genetic interactions including intragenic suppression, synthetic phenotypes, and intragenic complementation. These results support a
model of Mdm1p function in which a network comprised of multimeric assemblies of the protein mediates
two distinct cellular processes.
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