Reinvestigation of the functions of the hydrophobic segment of Jem1p, a yeast endoplasmic reticulum membrane protein mediating nuclear fusion.

Jem1p is a DnaJ-like protein of the yeast ER membrane, which is required for nuclear membrane fusion during mating. We have determined the position of translation initiation codon for the JEM1 gene. Translation of Jem1p starts from the second ATG codon of the previously assumed JEM1 open reading frame, leading to the synthesis of a precursor protein of 645 amino acids long. The translated Jem1p precursor contains an N-terminal hydrophobic sequence that functions as a signal sequence and is removed upon import into the ER lumen. Jem1p is peripherally associated with the ER membrane probably through protein-protein interactions.