A NOVEL FISSION YEAST GENE, THT1(+), IS REQUIRED FOR THE FUSION OF NUCLEAR ENVELOPES DURING KARYOGAMY

We have isolated a fission yeast karyogamy mutant, tht1, in which nucl ear congression and the association of two spindle pole bodies occurs but the subsequent fusion of nuclear envelopes is blocked, The tht1 mu tation does not prevent meiosis, so cells execute meiosis with two unf used nuclei, leading to the production of aberrant asci, The tht1(+) g ene was cloned and sequenced. Predicted amino acid sequence has no sig nificant homology to previously known proteins but strongly suggests t hat it is a type I membrane protein. The tht1(+) gene is dispensable f or vegetative growth and expressed only in conjugating cells. Tht1p is a glycoprotein susceptible to endoglycosilase H digestion. Site-direc ted mutagenesis showed that the N-glycosylation site, as well as the C OOH terminal region of Tht1p, is essential for its function, A proteas e protection assay indicated that the COOH terminus is cytoplasmic. Im munocytological analysis using a HA-tagged Tht1p suggested that the pr otein is localized in nuclear envelopes and in the ER during karyogamy and that its levels are reduced in cells containing fused nuclei.