DEVIANT PEX3P LEVELS AFFECT NORMAL PEROXISOME FORMATION IN HANSENULA-POLYMORPHA - HIGH STEADY-STATE LEVELS OF THE PROTEIN FULLY ABOLISH MATRIX PROTEIN IMPORT

PEX3 encodes a 52 kDa peroxisomal membrane protein (PNP), essential fo r peroxisome biogenesis in the yeast Hansenula polymorpha. The relatio n between Pex3p levels and peroxisome formation was studied in wild ty pe (WT) and Delta pex3 strains expressing additional copies of PEX3 un der control of a substrate-inducible promoter, namely the strong alcoh ol oxidase (P-AOX) or the weaker amine oxidase (P-AMO) promoter. In gl ucose-grown Delta pex3 cells, containing P-AOX.PEX3, Pex3p was undetec table and peroxisomes were absent. After induction of these cells on m ethanol, peroxisomes were rapidly formed. At Pex3p levels up to 7-10 t imes the values observed in WT controls normal peroxisomes were presen t. However, at further-enhanced Pex3p? levels a general matrix protein import defect was observed. This phenomenon was paralleled by aberran t peroxisome assembly and the formation of numerous small vesicles. Th ese vesicles contained Pex3p, together with other H. polymorpha PMPs, but lacked the major matrix proteins which has accumulated in the cyto sol. The implications of our results on PEX3 gene regulation and funct ioning of the peroxisomal matrix protein import machinery in H. polymo rpha are discussed. (C) 1997 John Wiley & Sons, Ltd.