p97 and close encounters of every kind: a brief review.

The AAA (ATPase associated with various cellular activities) ATPase, p97, is a hexameric protein of chaperone-like function, which has been reported to interact with a number of proteins of seemingly unrelated functions. For the first time, we report a classification of these proteins and aim to elucidate any common structural or functional features they may share. The interactors are grouped into those containing ubiquitin regulatory X domains, which presumably bind to p97 in the same way as the p47 adaptor, and into non-ubiquitin regulatory X domain proteins of different functional subgroups that may employ a different mode of interaction (assuming they also bind directly to p97 and are not experimental artifacts). Future studies will show whether interacting proteins direct p97 to different cellular pathways or a common one and structural elucidation of these interactions will be crucial in understanding these underlying functions.

Translocation pathway of protein substrates in ClpAP protease.

Intracellular protein degradation, which must be tightly controlled to protect normal proteins, is carried out by ATP-dependent proteases. These multicomponent enzymes have chaperone-like ATPases that recognize and unfold protein substrates and deliver them to the proteinase components for digestion. In ClpAP, hexameric rings of the ClpA ATPase stack axially on either face of the ClpP proteinase, which consists of two apposed heptameric rings. We have used cryoelectron microscopy to characterize interactions of ClpAP with the model substrate, bacteriophage P1 protein, RepA. In complexes stabilized by ATPgammaS, which bind but do not process substrate, RepA dimers are seen at near-axial sites on the distal surface of ClpA. On ATP addition, RepA is translocated through approximately 150 A into the digestion chamber inside ClpP. Little change is observed in ClpAP, implying that translocation proceeds without major reorganization of the ClpA hexamer. When translocation is observed in complexes containing a ClpP mutant whose digestion chamber is already occupied by unprocessed propeptides, a small increase in density is observed within ClpP, and RepA-associated density is also seen at other axial sites. These sites appear to represent intermediate points on the translocation pathway, at which segments of unfolded RepA subunits transiently accumulate en route to the digestion chamber.

Enzymatic and structural similarities between the Escherichia coli ATP-dependent proteases, ClpXP and ClpAP.

Escherichia coli ClpX, a member of the Clp family of ATPases, has ATP-dependent chaperone activity and is required for specific ATP-dependent proteolytic activities expressed by ClpP. Gel filtration and electron microscopy showed that ClpX subunits (Mr 46, 000) associate to form a six-membered ring (Mr approximately 280, 000) that is stabilized by binding of ATP or nonhydrolyzable analogs of ATP. ClpP, which is composed of two seven-membered rings stacked face-to-face, interacts with the nucleotide-stabilized hexamer of ClpX to form a complex that could be isolated by gel filtration. Electron micrographs of negatively stained ClpXP preparations showed side views of 1:1 and 2:1 ClpXP complexes in which ClpP was flanked on either one or both sides by a ring of ClpX. Thus, as was seen for ClpAP, a symmetry mismatch exists in the bonding interactions between the seven-membered rings of ClpP and the six-membered rings of ClpX. Competition studies showed that ClpA may have a slightly higher affinity (approximately 2-fold) for binding to ClpP. Mixed complexes of ClpA, ClpX, and ClpP with the two ATPases bound simultaneously to opposite faces of a single ClpP molecule were seen by electron microscopy. In the presence of ATP or nonhydrolyzable analogs of ATP, ClpXP had nearly the same activity as ClpAP against oligopeptide substrates (>10,000 min-1/tetradecamer of ClpP). Thus, ClpX and ClpA interactions with ClpP result in structurally analogous complexes and induce similar conformational changes that affect the accessibility and the catalytic efficiency of ClpP active sites.

At sixes and sevens: characterization of the symmetry mismatch of the ClpAP chaperone-assisted protease.

ClpAP, a typical energy-dependent protease, consists of a proteolytic component (ClpP) and a chaperone-like ATPase (ClpA). ClpP is composed of two apposed heptameric rings, whereas in the presence of ATP or ATPgammaS, ClpA is a single hexameric ring. Formation of ClpAP complexes involves a symmetry mismatch as sixfold ClpA stacks axially on one or both faces of sevenfold ClpP. We have analyzed these structures by cryo-electron microscopy. Our three-dimensional reconstruction of ClpA at 29-A resolution shows the monomer to be composed of two domains of similar size that, in the hexamer, form two tiers enclosing a large cavity. Cylindrical reconstruction of ClpAP reveals three compartments: the digestion chamber inside ClpP; a compartment between ClpP and ClpA; and the cavity inside ClpA. They are connected axially via narrow apertures, implying that substrate proteins should be unfolded to allow translocation into the digestion chamber. The cavity inside ClpA is structurally comparable to the "Anfinsen cage" of other chaperones and may play a role in the unfolding of substrates. A geometrical description of the symmetry mismatch was obtained by using our model of ClpA and the crystal structure of ClpP (Wang et al., 1997, Cell 91, 447-456) to identify the particular side views presented by both molecules in individual complexes. The interaction is characterized by a key pair of subunits, one of each protein. A small turn (8.6(o) = 2pi/42; equivalent to a 4-A shift) would transfer the key interaction to another pair of subunits. We propose that nucleotide hydrolysis results in rotation, facilitating the processive digestion of substrate proteins.

Aquaporin-related proteins in the filter chamber of homopteran insects.

In the Homopteran order of insects, the plant xylem feeders exhibit a highly differentiated part of their digestive tract known as the filter chamber. In this tissue, water crosses plasma membranes through a transepithelial osmotic gradient. In previous studies on the filter chamber of Cicadella viridis, we purified and characterized from the plasma membranes a 25 kDa protein that we demonstrated to be an aquaporin (or water channel, member of the major intrinsic protein family, a group of membrane channels for small solutes). We called this protein AQPcic for Cicadella aquaporin. In the present study, we used polyclonal antibody anti-AQPcic in Western blotting and immunocytochemical analysis of the intestinal tract of Cercopis sanguinolenta, Philaenus spumarius, Aphrophora alni (Cercopidae), Euscelidius variegatus, and Scaphoideus titanus (Jassidae). Western blotting experiments revealed that immunologically related AQPcic proteins are found in those species. The molecular weight of these proteins is 15-26 kDa. Immunocytochemical studies on ultrathin filter-chamber sections revealed that the anti-AQPcic antibody systematically labelled the membrane microvilli of epithelial cells. A good correlation thus exists between the physiology of these cells and the presence of aquaporin-related proteins in their membranes.

Structural analysis of a MIP family protein from the digestive tract of Cicadella viridis.

Homopteran insects, and especially Cicadella viridis, display in their digestive tract a specialized epithelial differentiation, the filter chamber (FC) acting as a water-shunting complex. The main intrinsic membrane protein of the FC is a 25,000-Da polypeptide (P25). In this paper we demonstrate that this P25 polypeptide is a member of the MIP family of membrane channel proteins, and that P25 forms homotetramers in the native membranes. Using polymerase chain reaction, a 360-base pair cDNA, named cic, was isolated from RNA of the FC. cic encodes a 119-amino acid polypeptide (CIC) whose homologies with MIP26, AQP1 (CHIP), AQP2, and gamma-TIP are 38, 38, 34, and 20%, respectively. Using a specific antibody raised against a 15-amino acid peptide from the CIC sequence, we concluded that CIC and P25 are identical entities, and hence that P25 belongs to the MIP family. We investigated the quaternary structure of P25 in the membranes of the FC using biophysical analysis of P25 nondenaturing detergent micelles, scanning transmission electron microscopy, and image processing of conventional transmission electron microscopic images. All those different approaches converged to the conclusion that P25 exists as an homotetramer forming a regular two-dimensional array in the membranes.

Expression of RNA isolated from the water-shunting complex of a sap-sucking insect increases the membrane permeability for water in Xenopus oocytes.

The highly specialized membranes of the filter chamber found in the digestive tract of some homopteran insects could represent a favorable material for characterizing water channels. In order to demonstrate that membrane proteins of this epithelial complex serve as water channels, we have investigated the membrane permeability for water in Xenopus oocytes injected with RNA isolated from the filter chamber. Volumes of oocytes injected with filter chamber RNA were increased by 15% following a 16-min osmotic shock, while volumes of oocytes injected with RNA from midgut not of filter chamber or with water were increased only by 8.5 and 10%, respectively. This significant difference in oocyte swelling leads us to conclude that RNA isolated from the filter chamber contains mRNA coding for water channel proteins.