The Channel-Forming Sym1 Protein Is Transported by the TIM23 Complex in a Presequence-Independent Manner

Abstract

fThe majority of multispanning inner mitochondrial membrane proteins utilize internal targeting signals, which direct them to the carrier translocase (TIM22 complex), for their import. MPV17 and its Saccharomyces cerevisiae orthologue Sym1 are multispanning inner membrane proteins of unknown function with an amino-terminal presequence that suggests they may be targeted to the mitochondria. Mutations affecting MPV17 are associated with mitochondrial DNA depletion syndrome (MDDS). Reconstitution of purified Sym1 into planar lipid bilayers and electrophysiological measurements have demonstrated that Sym1 forms a membrane pore. To address the biogenesis of Sym1, which oligomerizes in the inner mitochondrial membrane, we studied its import and assembly pathway. Sym1 forms a transport intermediate at the translocase of the outer membrane (TOM) complex. Surprisingly, Sym1 was not transported into mitochondria by an amino-terminal signal, and in contrast to what has been observed in carrier proteins, Sym1 transport and assembly into the inner membrane were independent of small translocase of mitochondrial inner membrane (TIM) and TIM22 complexes. Instead, Sym1 required the presequence of translocase for its biogenesis. Our analyses have revealed a novel transport mechanism for a polytopic membrane protein in which internal signals direct the precursor into the inner membrane via the TIM23 complex, indicating a presequence-independent function of this translocase.

The Human Mitochondrial DNA Depletion Syndrome Gene MPV17 Encodes a Non-selective Channel That Modulates Membrane Potential

“…Recently, the information about channelforming activity of the yeast inner mitochondrial membrane protein Sym1 homologous to mammalian Mpv17 and Pxmp2 proteins was published (16). These data together with our observations (this work) indicate that both peroxisomal (Pxmp2 (11)) and mitochondrial (fungal Sym1 (16) as well as mouse Mpv17 (15) and human MPV17) members of the Pxmp2 protein family are in fact membrane channels.…”

“…All of them are cation-selective channels with a large pore size (1.4 and 1.6 nm for Pxmp2 and Sym1, respectively) (11,16). Under standard redox conditions (without the addition of reducing agents), the channels are resistant to closing at moderate membrane potentials.…”

“…However, the precise function of Mpv17 in mitochondria has not been established. Our recent observations with mouse Mpv17 (15) and the detection of the pore-forming activity of Sym1, the yeast ortholog of MPV17 (16), suggested that human MPV17 may also have a role as a non-selective channel. These findings, however, appeared to be counterintuitive because the localization of open non-selective channels in the inner mitochondrial membrane would not favor the preservation of a high membrane potential (⌬ m ) under physiological conditions.…”

Mcp1 and Mcp2, two novel proteins involved in mitochondrial lipid homeostasis

“…According to the Saccharomyces Genome Database (SGD) (www.yeastgenome.org), NDL1 has no obvious connection to mitochondrial function, and YLR252W is a dubious ORF. SYM1 encodes for a transporter of the inner mitochondrial membrane (Trott and Morano, 2004;Dallabona et al, 2010;Reinhold et al, 2012) with no obvious link to Mdm10 or any of its proposed functions. Of note, YLR253W encodes for a yet uncharacterized protein that was detected in the mitochondrial proteome (Reinders et al, 2006).…”

Alternative Splicing-Mediated Targeting of the Arabidopsis GLUTAMATE RECEPTOR3.5 to Mitochondria Affects Organelle Morphology

“…The observation that the expression of AtGLR3.5 increases with aging would be in accordance with the above hypothesis. In this respect, it is interesting to mention the case of stress-induced yeast (Saccharomyces cerevisiae) ortholog of the mammalian kidney disease gene Mpv17 (Sym1), a channel-forming protein (Reinhold et al, 2012) that is the yeast homolog of MPV17, a mammalian mitochondrial inner membrane protein, whose mutations are associated with and inherited autosomal mitochondrial disease characterized by neuronal disorders, growth retardation, liver failure, and hypoglycemia (e.g. Szabò and Zoratti, 2014).…”