Redox-sensitive Homodimerization of Pexl lp: A Proposed Mechanism to Regulate Peroxisomal Division

P e x l l p (formerly Pmp27) has been implicated in peroxisomal proliferation (Erdmann, R., and G. Blobel. 1995. J. Cell Biol. 128: 509-523; Marshall, P.A., Y.I. Krimkevich, R.H. Lark, J.M. Dyer, M. Veenhuis, and J.M. Goodman. 1995. J. Cell Biol. 129: 345355). In its absence, peroxisomes in Saccharomyces cerevisiae fail to proliferate in response to oleic acid; instead, one or two large peroxisomes are formed. Conversely, overproduction of Pexl lp causes an increase in peroxisomal number. In this report, we confirm the function of P e x l l p in organelle proliferation by demonstrating that this protein can cause fragmentation in vivo of large peroxisomes into smaller organelles. P e x l l p is on the inner surface of the peroxisomal membrane. It can form homodimers, and this species is more abundant in mature peroxisomes than in proliferating organelles. Removing one of the three cysteines in the protein inhibits homodimerization. This cysteine 3 --~ alanine mutation leads to an increase in number and a decrease in peroxisomal density, compared with the wild-type protein, in response to oleic acid. We propose that the active species is the "monomeric" form, and that the increasing oxidative metabolism within maturing peroxisomes causes dimer formation and inhibition of further organelle division. T HE division of organelles is a basic cell function. It can be useful to think of organellar division as either constitutive or regulated, borrowing the nomenclature from the secretion field. Constitutive division is required to maintain organelle number and volume as cells divide and organelles age and undergo autophagy. In contrast, organelles can undergo regulated division in response to external signals or to internal programming. Thus, exposure to barbituates or overexpression of 3-hydroxy-3methyl-glutaryl coenzyme A reductase causes the proliferation of the endoplasmic reticulum (Jingami et al., 1987; Michalopoulos et al., 1976). Peroxisomes are also exquisitely sensitive to external signals. In animal cells, various hypolipidemic drugs and plasticizers cause proliferation of this organelle (Lock et al., 1989), probably as downstream events upon activation of peroxisome proliferator-activated receptor-a (Lee et al., 1995). Although not all peroxisomal enzymes are induced by these agents, there is a severalfold increase in organelle number which accommodates an increased capacity for fatty acid 13-oxidation (reviewed in Green, 1995). Peroxisomal number and size are regulated to an even stronger extent in the methylotrophic yeasts which, for many years, have been model systems for peroxisomal asAddress all correspondence to Joel M. Goodman, Department of Pharmacology, University of Texas Southwestern Medical Center, 5323 Harry Hines Boulevard, Dallas, TX 75235-9041. Tel.: (214) 648-2359. Fax: (214) 648-2994. e-mail: [email protected] sembly. For example, Candida boidinii grown on glucose medium contain only a few very small peroxisomes which undergo constitutive division as cells divide. When these cells are grown in the presence of either methanol, fatty acids, or D-alanine, however, there is massive peroxisomal proliferation, such that 30-50% of cell volume becomes comprised of peroxisomes, either clustered or separated (Goodman et al., 1990). The enzyme content of these organelles depends on growth substrate, because these compounds are directly utilized for peroxisomal metabolism, and the enzymes of different pathways are tightly regulated by the growth substrate (Veenhuis and Harder, 1987). In contrast to matrix proteins, peroxisomes of C. boidinii contain membrane proteins that are induced by all proliferation substrates (Goodman et al., 1990). One such protein is Pexllp, also termed peroxin 11. (This protein was originally termed Pmp31 and Pmp32 to designate two nearly identical isoforms in C. boidinii [Goodman et al., 1986; Moreno et al., 1994], and later Pmp30A and B [Sakai et al., 1995]). The orthologue in Saccharomyces cerevisiae was termed Pmp24 (McCammon et al., 1990), later Pmp27 (Erdmann and Blobel, 1995; Marshall et al., 1995). The proteins in these two yeast species (CbPexl lp and ScPexllp) can cross-complement (Sakai et al., 1995). Pexl lp associates with the peroxisomal membrane but is releasable at high pH, indicating that it does not cross the bilayer (Goodman et al., 1986; Marshall et al., 1995). We and others have shown that Pexl lp functions in reg© The Rockefeller University Press, 0021-9525/96/10/123/15 $2.00 The Journal of Cell Biology, Volume 135, Number 1, October 1996 123-137 123 on F ebuary 1, 2013 jcb.rress.org D ow nladed fom Published October 1, 1996