OVEREXPRESSION OF PEX15P, A PHOSPHORYLATED PEROXISOMAL INTEGRAL MEMBRANE-PROTEIN REQUIRED FOR PEROXISOME ASSEMBLY IN SACCHAROMYCES-CEREVISIAE, CAUSES PROLIFERATION OF THE ENDOPLASMIC-RETICULUM MEMBRANE

We have cloned PEX15 which is required for peroxisome biogenesis in Sa ccharomyces cerevisiae. pex15 Delta cells are characterized by the cyt osolic accumulation of peroxisomal matrix proteins containing a PTS1 o r PTS2 import signal, whereas peroxisomal membrane proteins are presen t in peroxisomal remnants, PEX15 encodes a phosphorylated, integral pe roxisomal membrane protein (Pex15p). Using multiple in vivo methods to determine the topology, Pex15p was found to be a tail-anchored type I I (N-cyt-C-lumen) peroxisomal membrane protein with a single transmemb rane domain near its carboxy-terminus. Overexpression of Pex15p result ed in impaired peroxisome assembly, and caused profound proliferation of the endoplasmic reticulum (ER) membrane, The lumenal carboxy-termin al tail of Pex15p protrudes into the lumen of these ER membranes, as d emonstrated by its O-glycosylation, Accumulation in the ER was also ob served at an endogenous expression level when Pex15p was fused to the N-terminus of mature invertase, This resulted in core N-glycosylation of the hybrid protein, The lumenal C-terminal tail of Pex15p is essent ial for targeting to the peroxisomal membrane, Furthermore, the peroxi somal membrane targeting signal of Pex15p overlaps with an ER targetin g signal on this protein, These results indicate that Pex15p may be ta rgeted to peroxisomes via the ER, or to both organelles.