Partial characterization of mitochondrial G proteins in adrenal cells

Novel roles for the rho subfamily of GTP-binding proteins in succinate-induced insulin secretion from betaTC3 cells: further evidence in support of the succinate mechanism of insulin release

We have previously demonstrated regulatory roles for Rho subfamily of G-proteins in glucose- and calcium-induced insulin secretion. Herein, we examined regulation by these proteins of insulin secretion from betaTC3 cells elicited by mitochondrial fuels, such as the succinic acid methyl ester (SAME). Preincubation of these cells with Clostridium difficile toxin-B (200 ng/mL), which monoglucosylates and inactivates Cdc42 and Rac1, markedly decreased (> 70%) SAME-induced insulin secretion. Furthermore, exposure of betaTC3 cells to GGTI-2147 (20 microM), a selective inhibitor of the requisite prenylation of Rac1 and Cdc42, significantly reduced (> 80%) SAME-induced insulin release, suggesting that post-translational prenylation of these proteins is necessary for SAME-induced insulin release. Western blot analysis indicated localization of Cdc42, Rac1, and Ras in the beta cell mitochondrial fraction. Confocal microscopy revealed a modest, but inconsistent, increase in the association of either Rac1 or Cdc42 with Mitotracker, a mitochondrial marker, following exposure to SAME. These data suggest that activation of preexisting intramitochondrial Rac1 and Cdc42 may be sufficient to regulate SAME-induced insulin secretion. Together, our findings support a role for G-proteins in insulin secretion at a step dependent on mitochondrial metabolism. They also identify mevalonate-derived, isoprenoid modified Rho G-proteins as specific signaling molecules in recently proposed succinate mechanism of insulin release.

The regulation of mitochondrial physiology by organelle-associated GTP-binding proteins

Recent studies have shown that GTP-binding proteins can modulate mitochondrial membrane fusion and fission. Furthermore, GTP-binding proteins can regulate the binding of ribosomes to the mitochondrial membrane and may facilitate the import of proteins through contact points between inner and outer mitochondrial membranes. Mitochondrial GTP-binding proteins therefore appear to have the potential to modulate physiological function of the organelle and may also be involved in cellular processes such as cellular transformation. A beginning has been made on the characterization of mitochondrial GTP-binding proteins and the DNA sequence of one protein has become newly available. Future studies are needed to determine whether GTP-binding proteins are interacting with cell signalling molecules such as protein kinases in the mitochondria.

The Xq22 inversion breakpoint interrupted a novel Ras-like GTPase gene in a patient with Duchenne muscular dystrophy and profound mental retardation

A male patient with profound mental retardation, athetosis, nystagmus, and severe congenital hypotonia (Duchenne muscular dystrophy [DMD]) was previously shown to carry a pericentric inversion of the X chromosome, 46,Y,inv(X)(p21.2q22.2). His mother carried this inversion on one X allele. The patient's condition was originally misdiagnosed as cerebral palsy, and only later was it diagnosed as DMD. Because the DMD gene is located at Xp21.2, which is one breakpoint of the inv(X), and because its defects are rarely associated with severe mental retardation, the other clinical features of this patient were deemed likely to be associated with the opposite breakpoint at Xq22. Our precise molecular-cytogenetic characterization of both breakpoints revealed three catastrophic genetic events that had probably influenced neuromuscular and cognitive development: deletion of part of the DMD gene at Xp21.2, duplication of the human proteolipid protein gene (PLP) at Xq22.2, and disruption of a novel gene. The latter sequence, showing a high degree of homology to the Sec4 gene of yeast, encoded a putative small guanine-protein, Ras-like GTPase that we have termed "RLGP." Immunocytochemistry located RLGP at mitochondria. We speculate that disruption of RLGP was responsible for the patient's profound mental retardation.

ARL2 and BART enter mitochondria and bind the adenine nucleotide transporter

The ADP-ribosylation factor-like 2 (ARL2) GTPase and its binding partner binder of ARL2 (BART) are ubiquitously expressed in rodent and human tissues and are most abundant in brain. Both ARL2 and BART are predominantly cytosolic, but a pool of each was found associated with mitochondria in a protease-resistant form. ARL2 was found to lack covalent N-myristoylation, present on all other members of the ARF family, thereby preserving the N-terminal amphipathic alpha-helix as a potential mitochondrial import sequence. An overlay assay was developed to identify binding partners for the BART.ARL2.GTP complex and revealed a specific interaction with a protein in bovine brain mitochondria. Purification and partial microsequencing identified the protein as an adenine nucleotide transporter (ANT). The overlay assay was performed on mitochondria isolated from five different tissues from either wild-type or transgenic mice deleted for ANT1. Results confirmed that ANT1 is the predominant binding partner for the BART.ARL2.GTP complex and that the structurally homologous ANT2 protein does not bind the complex. Cardiac and skeletal muscle mitochondria from ant1(-)/ant1(-) mice had increased levels of ARL2, relative to that seen in mitochondria from wild-type animals. We conclude that the amount of ARL2 in mitochondria is subject to regulation via an ANT1-sensitive pathway in muscle tissues.