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J. Cell Biol.,
Volume 142, Number 3, August 10, 1998 613-623
Department of Molecular Biology and Oncology, University of Texas Southwestern Medical Center, Dallas, Texas 75235-9148
Green fluorescent protein (GFP) was used to
tag proteins of the mitochondrial matrix, inner, and
outer membranes to examine their sorting patterns relative to mtDNA in zygotes of synchronously mated
yeast cells in 
+ × 0 crosses. When transiently expressed in one of the haploid parents, each of the
marker proteins distributes throughout the fused mitochondrial reticulum of the zygote before equilibration
of mtDNA, although the membrane markers equilibrate slower than the matrix marker. A GFP-tagged
form of Abf2p, a mtDNA binding protein required for faithful transmission of + mtDNA in vegetatively
growing cells, colocalizes with mtDNA in situ. In zygotes of a + × + cross, in which there is little mixing
of parental mtDNAs, Abf2p-GFP prelabeled in one
parent rapidly equilibrates to most or all of the
mtDNA, showing that the mtDNA compartment is accessible to exchange of proteins. In + × 0 crosses,
mtDNA is preferentially transmitted to the medial diploid bud, whereas mitochondrial GFP marker proteins
distribute throughout the zygote and the bud. In zygotes lacking Abf2p, mtDNA sorting is delayed and
preferential sorting is reduced. These findings argue for
the existence of a segregation apparatus that directs mtDNA to the emerging bud.
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