Required for the fusion of nuclear envelopes during mating, ensuring proper karyogamy. Plays a role in the initiation of outer nuclear envelope fusion. Interacts with KAR5. By pheromone. Expression is controlled by the STE12 transcription factor that binds to pheromone-response cis-elements (PREs) in the promoter of target genes. Was originally shown to be localized in the inner nuclear membrane by using a truncated protein (PubMed:12514182). Was used as an inner nuclear membrane marker (PubMed:17996101, PubMed:18660802 and PubMed:19369416). However, more recent studies showed that it is localized in the nuclear outer membrane, which is consistent with a function in the initiation of nuclear fusion (PubMed:19297527 and PubMed:19570912).