Abstract
The conserved CaaX box peroxin Pex19p is known to be modified by farnesylation. The possible involvement of this lipid modification in peroxisome biogenesis, the degree to which Pex19p is farnesylated, and its molecular function are unknown or controversial. We resolve these issues by first showing that the complete pool of Pex19p is processed by farnesyltransferase in vivo and that this modification is independent of peroxisome induction or the Pex19p membrane anchor Pex3p. Furthermore, genomic mutations of PEX19 prove that farnesylation is essential for proper matrix protein import into peroxisomes, which is supposed to be caused indirectly by a defect in peroxisomal membrane protein (PMP) targeting or stability. This assumption is corroborated by the observation that mutants defective in Pex19p farnesylation are characterized by a significantly reduced steady-state concentration of prominent PMPs (Pex11p, Ant1p) but also of essential components of the peroxisomal import machinery, especially the RING peroxins, which were almost depleted from the importomer. In vivo and in vitro, PMP recognition is only efficient when Pex19p is farnesylated with affinities differing by a factor of 10 between the non-modified and wild-type forms of Pex19p. Farnesylation is likely to induce a conformational change in Pex19p. Thus, isoprenylation of Pex19p contributes to substrate membrane protein recognition for the topogenesis of PMPs, and our results highlight the importance of lipid modifications in protein-protein interactions.
Highlights
The farnesyl group is attached to the cysteine of the C-terminal motif known as the CaaX box, where “a” indicates aliphatic amino acids and X is usually serine, methionine, glutamine, alanine, or threonine [3]
We discovered that Pex19p is fully modified by FTase and investigated whether Pex19p farnesylation is required for peroxisomal membrane proteins (PMPs) recognition and stability
All Pex19p Is Processed by Farnesyltransferase—To determine the level of Pex19p farnesylation in vivo, we analyzed cell lysates of S. cerevisiae wild-type cells by immunoblotting with antibodies directed against Pex19p
Summary
Oligonucleotides, Plasmids, and Strains—Oligonucleotides and plasmids are listed in supplemental Tables 1 and 2. Immunoprecipitation of Protein A-tagged Pex2p was performed as described in Ref. 32. Protein Expression—GST-Pex19p and GST-Ras1p were expressed in Escherichia coli and purified as described for GSTPex19p [18]. Pex19p in vitro binding assay with peptide arrays was carried out essentially as described [18]. Concentrations of free and bound Pex19p or Pex19pFARN were calculated from the starting concentrations of Pex19p and peptide applied and the amplitude of the titration curve. For both titration experiments, data were fitted using GraFit version 3.0 software (Erithacus) according to an A ϩ B ϭ AB binding model.
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