Poster: Organelle Biogenesis

Abs # 1162: Two distinct (non-overlapping) targeting signals sort AtPex16p to peroxisomes indirectly via a pre-peroxisomal compartment.

Presenter:	Karnik, Sheetal K, skarnik@asu.edu
Authors	Karnik, Sheetal K (A)
Affiliations:	(A): Molecular and Cellular Biology Program, Department of Plant Biology, Arizona State University

Peroxins (Pex) are proteins involved in biogenesis of peroxisomes. In a study of the Arabidopsis shrunken seed SSE1 mutant, Lin et al., (Science 284:328, 1999) reported that the AtPex16 ortholog was involved in protein and oil body biogenesis from ER; no link was made to peroxisomal biogenesis. We found via immunofluorescence microscopy, however, that transiently-expressed myc-AtPex16p sorted to peroxisomes and/or a reticular compartment (not colocalized with calnexin) in BY2 and Arabidopsis cultured cells. Sorting to peroxisomes was blocked with brefeldinA or when held at 15°C; instead, AtPex16p accumulated in a reticular compartment, which colocalized with calnexin. After removal of brefeldinA, or raising the temperature, AtPex16p sorted to peroxisomes. Hence, this reticular compartment may be a pre-peroxisomal compartment (PPC), possibly a sub-domain of ER. AtPex16p (367 amino acids), has two internal transmembrane domains (TMDs), each with tri-arginine repeats in close proximity. AtPex16p is oriented in the reticular and the peroxisomal membrane with both N and C terminal ends facing the cytosol. The necessary targeting signals for At Pex16p to the peroxisomal membranes lie within the second transmembrane domain (TMD2) along with the downstream tri-arginine residues. The signals necessary for sorting to the PPC lie within the first transmembrane domain (TMD1) along with the upstream tri-arginine residues. In summary, our data indicates that AtPex16p sorts to PPC (ER subdomain?) and then to peroxisomes, and the targeting signals for these two compartments are internal and nonoverlapping. Supported by NSF grant MCB-0091826.