Deviant Pex3p Levels affect Normal Peroxisome Formation in Hansenula polymorpha: A Sharp Increase of the Protein Level Induces the Proliferation of Numerous, Small Protein‐import Competent Peroxisomes

Pex3p has been implicated in the biosynthesis of the peroxisomal membrane of the yeast Hansenula polymorpha. Here we show that in the initial stages of a sharp increase in Pex3p levels, induced in batch cultures of cells of a constructed H. polymorpha strain, which contained seven copies of PEX3 under control of the alcohol oxidase promoter (WT::PAOX.PEX37x), strongly interfered with normal peroxisome proliferation. Ultrastructural studies demonstrated that in such cells numerous small peroxisomes had developed, which were absent in wild‐type controls. These organelles, which contained typical peroxisomal matrix and membrane proteins (alcohol oxidase, catalase, Pex3p, Pex10p and Pex14p), showed a relatively low density (1·18 g cm−3) after sucrose gradient centrifugation of WT::PAOX.PEX37x homogenates, compared to normal peroxisomes (1·23 g cm−3). We furthermore demonstrated that these early induced, small peroxisomes were protected against glucose‐induced proteolytic degradation and did not fuse to form larger organelles. Remarkably, the induction of these small peroxisomes was paralleled by a partial defect in matrix protein import, reflected by the mislocalization of minor amounts of alcohol oxidase protein in the cytosol. However, when the cells were subsequently placed under conditions in which the synthesis of a new matrix enzyme (amine oxidase) was induced while simultaneously the excessive proliferation was repressed (by repression of the PAOX), amine oxidase protein was selectively incorporated into these organelles. This indicated that the small peroxisomes had regained a normal protein import capacity. Based on these results we argue that peroxisome proliferation and matrix protein import are coupled processes in H. polymorpha. © 1997 John Wiley & Sons, Ltd.