The mitochondrial prohibitin complex is essential for embryonic viability and germline function in Caenorhabditis elegans

Prohibitins in eukaryotes consist of two subunits (PHB1 and PHB2) that together form a high molecular weight complex in the mitochondrial inner membrane. The evolutionary conservation and the ubiquitous expression in mammalian tissues of the prohibitin complex suggest an important function among eukaryotes. The PHB complex has been shown to play a role in the stabilization of newly synthesized subunits of mitochondrial respiratory enzymes in the yeast Saccharomyces cerevisiae. We have used Caenorhabditis elegans as model system to study the role of the PHB complex during development of a multicellular organism. We demonstrate that prohibitins in C. elegans form a high molecular weight complex in the mitochondrial inner membrane similar to that of yeast and humans. By using RNA-mediated gene inactivation, we show that PHB proteins are essential during embryonic development and are required for somatic and germline differentiation in the larval gonad. We further demonstrate that a deficiency in PHB proteins results in altered mitochondrial biogenesis in body wall muscle cells. This paper reports a strong loss of function phenotype for prohibitin gene inactivation in a multicellular organism and shows for the first time that prohibitins serve an essential role in mitochondrial function during organismal development.This work was supported by European Commission Grant QLG1-CT-2001-00966.Peer reviewe

Prohibitins act as a membrane-bound chaperone for the stabilization of mitochondrial proteins

Prohibitins are ubiquitous, abundant and evolutionarily strongly conserved proteins that play a role in important cellular processes. Using blue native electrophoresis we have demonstrated that human prohibitin and Bap37 together form a large complex in the mitochondrial inner membrane. This complex is similar in size to the yeast complex formed by the homologues Phb1p and Phb2p. In yeast, levels of this complex are increased on co-overexpression of both Phb1p and Phb2p, suggesting that these two proteins are the only components of the complex. Pulse–chase experiments with mitochondria isolated from phb1/phb2-null and PHB1/2 overexpressing cells show that the Phb1/2 complex is able to stabilize newly synthesized mitochondrial translation products. This stabilization probably occurs through a direct interaction because association of mitochondrial translation products with the Phb1/2 complex could be demonstrated. The fact that Phb1/2 is a large multimeric complex, which provides protection of native peptides against proteolysis, suggests a functional homology with protein chaperones with respect to their ability to hold and prevent misfolding of newly synthesized proteins

A structure for the yeast prohibitin complex: Structure prediction and evidence from chemical crosslinking and mass spectrometry

The mitochondrial prohibitin complex consists of two subunits (PHB1 of 32 kD and PHB2 of 34 kD), assembled into a membrane-associated supercomplex of approximately 1 MD. A chaperone-like function in holding and assembling newly synthesized mitochondrial polypeptide chains has been proposed. To further elucidate the function of this complex, structural information is necessary. In this study we use chemical crosslinking, connecting lysine side chains, which are well scattered along the sequence. Crosslinked peptides from protease digested prohibitin complexes were identified with mass spectrometry. From these results, spatial restraints for possible protein conformation were obtained. Many interaction sites between PHB1 and PHB2 were found, whereas no homodimeric interactions were observed. Secondary and tertiary structural predictions were made using several algorithms and the models best fitting the spatial restraints were selected for further evaluation. From the structure predictions and the crosslink data we derived a structural building block of one PHB1 and one PHB2 subunit, strongly intertwined along most of their length. The size of the complex implies that approximately 14 of these building blocks are present. Each unit contains a putative transmembrane helix in PHB2. Taken together with the unit building block we postulate a circular palisade-like arrangement of the building blocks projecting into the intermembrane space