THE METHYLOTROPHIC YEAST HANSENULA-POLYMORPHA CONTAINS AN INDUCIBLE IMPORT PATHWAY FOR PEROXISOMAL MATRIX PROTEINS WITH AN N-TERMINAL TARGETING SIGNAL (PTS2 PROTEINS)

Two main types of peroxisomal targeting signals have been identified t hat reside either at the extreme C terminus (PTS1) or the N terminus ( PTS2) of the protein. In the methylotrophic yeast Hansenula polymorpha the majority of peroxisomal matrix proteins are of the PTS1 type. Thu s far, for H. polymorpha only amine oxidase (AMO) has been shown to co ntain a PTS2 type signal. In the present study we expressed H. polymor pha AMO under control of the strong endogenous alcohol oxidase promote r. Partial import of AR IO into peroxisomes was observed in cells grow n in methanol/(NH4)(2)SO4-containing medium. However, complete import of AMO occurred if the cells were grown under conditions that induce e xpression of the endogenous AMO gene. Similar results were obtained wh en the heterologous PTS2 proteins, glyoxysomal malate dehydrogenase fr om watermelon and thiolase from Saccharomyces cerevisiae, were synthes ized in H. polymorpha. The import of PTS1 proteins, however, was not a ffected by the growth conditions. These results indicate that the redu ced rate of AMO import in (NH4)(2)SO4-grown cells is not due to compet ition with PTS1 proteins for the same import pathway. Apparently, AMO is imported via a separate pathway that is induced by amines and funct ions for PTS2 proteins in general.