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Vol. 20, Issue 9, 2438-2450, May 1, 2009

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Prm3p Is a Pheromone-induced Peripheral Nuclear Envelope Protein Required for Yeast Nuclear Fusion

Department of Molecular Biology, Princeton University, Princeton, NJ 08544-1014

Submitted October 2, 2008; Revised February 5, 2009; Accepted March 9, 2009
Monitoring Editor: Tim Stearns

Nuclear membrane fusion is the last step in the mating pathway of the yeast Saccharomyces cerevisiae. We adapted a bioinformatics approach to identify putative pheromone-induced membrane proteins potentially required for nuclear membrane fusion. One protein, Prm3p, was found to be required for nuclear membrane fusion; disruption of PRM3 caused a strong bilateral defect, in which nuclear congression was completed but fusion did not occur. Prm3p was localized to the nuclear envelope in pheromone-responding cells, with significant colocalization with the spindle pole body in zygotes. A previous report, using a truncated protein, claimed that Prm3p is localized to the inner nuclear envelope. Based on biochemistry, immunoelectron microscopy and live cell microscopy, we find that functional Prm3p is a peripheral membrane protein exposed on the cytoplasmic face of the outer nuclear envelope. In support of this, mutations in a putative nuclear localization sequence had no effect on full-length protein function or localization. In contrast, point mutations and deletions in the highly conserved hydrophobic carboxy-terminal domain disrupted both protein function and localization. Genetic analysis, colocalization, and biochemical experiments indicate that Prm3p interacts directly with Kar5p, suggesting that nuclear membrane fusion is mediated by a protein complex.

* Present address: Dartmouth Center for the Advancement of Learning (DCAL), 102 Baker-Berry Library, HB 6247, Hanover, NH 03755.

Address correspondence to: Mark D. Rose (mdrose{at}princeton.edu).

Abbreviations used: SPB, spindle pole body.

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