Functional Characterization of the Human Peroxins PEX3 and PEX19, Proteins Essential for Early Peroxisomal Membrane Biogenesis
Year: 2003
Type: dissertation
Abstract: Peroxisomes are single-membrane-bound organelles present in virtually all eukaryotic cells. Their significance in human metabolism is illustrated by the existence of severe inherited diseases caused by the failure of peroxisomal biogenesis (peroxisomal biogenesis disorders; PBDs). Proteins required for peroxisomal assembly are termed peroxins and are encoded by at least 23 PEX-genes. In most cases, defects in PEX genes lead to a disruption of peroxisomal matrix protein import, whereas various peroxisomal membrane components are synthesized and accumulate in peroxisomal membrane remnants (peroxisomal ghosts). Since yeast mutants in PEX3 and PEX19 have been shown to lack peroxisomal ghosts, these peroxins were expected to be involved in the early stages of peroxisomal membrane synthesis. This work focused on the functional characterization of human PEX3 and PEX19 to gain insights into their role in human biology and disease. PEX19 was shown to interact with three peroxisomal ABC half-transporters (Adrenoleukodystrophy protein ALDP, Adrenoleukodystrophy-related protein ALDRP, and the 70 kDa peroxisomal membrane protein PMP70), confirming its role as a broad specific peroxisomal membrane-protein binding-protein. As a prerequisite for these protein-protein interaction assays, the human ALDR gene was characterized. Proteins encoded by two PEX19 splice variants showed a considerable functional diversity as to peroxisomal membrane protein binding and with respect to induction of peroxisomal formation in PEX19-deficient human fibroblasts. Farnesylation of PEX19 was shown not to be essential for these functions. These data provide the first experimental evidence for specific biological functions of the different predicted domains of the PEX19 protein. In order to investigate the molecular details of peroxisomal assembly and to evaluate experimental treatment strategies for the peroxisomal biogenesis disorders, data useful for the future generation of a mouse model with targeted disruption of the PEX3 gene were presented. The human PEX3 gene was characterized with the goal to identify a yet unknown human PEX3-deficient phenotype. Two inactivating PEX3 mutations were detected in two PBDs-patients exhibiting a severe and lethal Zellweger syndrome phenotype. They showed a total lack of morphologically recognizable peroxisomal membrane remnants in their fibroblasts. Expression of the wild type PEX3 cDNA in the mutant cell lines restored peroxisomal biogenesis, establishing PEX3 as a key factor in early human peroxisome synthesis. Taken together, the combined data presented here provide evidence for the essential role of PEX3 and PEX19 in the initial steps of an alternative human peroxisome-formation pathway. In contrast to the theory that peroxisomes can arise exclusively by growth and division of preexisting peroxisomes, this alternative pathway does not require morphologically recognizable peroxisomal membranes.
Author: Peter Mayerhofer
Language: English
Subfield: Clinical Biochemistry
Domain: Life Sciences
Open Access status: green
OpenAlex ID: W2746749431