The complex of mitochondrial F1-ATPase with the natural inhibitor protein is unable to catalyze single-site ATP hydrolysis

Interaction of mitochondrial F₁-ATPase with the isolated natural inhibitor protein resulting in the inhibition of multi-site ATP hydrolysis is accompanied by the loss of activity at low ATP concentrations when single-site hydrolysis should occur. Catalytic site occupancy by [¹⁴C]nucleotides in F₁-ATPase during steady-state [¹⁴C]ATP hydrolysis, which is saturated in parallel with single-site catalysis, is prevented after blocking the enzyme with the inhibitor protein.     

Variant forms of the pig lutropin/choriogonadotropin receptor.

The cloning and sequencing of porcine lutropin/choriogonadotropin (LH/hCG) receptor messenger RNAs have shown the presence of a full-length receptor (pLHR-A) and of shorter variants lacking either the transmembrane and the intracellular domains (pLHR-B and pLHR-C) or only the transmembrane domain (pLHR-D). Moreover, immunoblotting of testicular membrane extracts has detected 85-, 68-, and 45-48-kDa proteins reacting with antireceptor antibodies. Transfection experiments were performed to assign the protein species to the various messenger RNAs and to study the function of the various receptor species. COS-7 and L-cells transfected with an expression vector encoding full-length receptor pLHR-A yielded a protein of apparent molecular mass of 105 kDa. This corresponded to the complete receptor which had undergone a different glycosylation pattern to that found in testis, since after digestion with peptide N-glycosidase F both the 105-kDa COS-7 protein and the 85-kDa testicular glycoprotein yielded a holoprotein of approximately 63 kDa. Transfection with pLHR-A also yielded a high proportion of the 68-kDa glycoprotein which was shown by digestion with endoglycosidase H to be a high-mannose precursor of the full-length receptor. The existence of a large pool of precursor species in both transfected cells and Leydig cells evokes possible physiological regulations at the level of receptor maturation.(ABSTRACT TRUNCATED AT 250 WORDS)     

Pathways of internalization of the hCG/LH receptor: immunoelectron microscopic studies in Leydig cells and transfected L-cells

Monoclonal anti-receptor antibodies were used to study the cellular traffic of the hCG/LH receptor by immunoelectron microscopy. The LHR38 antibody was shown to bind to the extracellular domain of the receptor but not to interfere with hormone binding, adenylate cyclase activation or with the rate of internalization of the receptor. Pig Leydig cells and a permanent L-cell line expressing the LH receptor were used for the study. Incubation with LHR38-gold complexes showed the LH receptors to be randomly distributed over the cell surface including the clathrin coated pits. The LH receptors were internalized via a route including coated pits, coated vesicles and multivesicular bodies to lysosomes. This route is different from that observed for beta-adrenergic, muscarinic, and yeast mating factor receptors and considered previously as possibly general for G-protein-coupled receptors. The use of [125I]LHR38 allowed precise measurement of the rate of internalization, showing the existence of a constitutive pathway which was increased 11-fold by hormone administration. Double labeling experiments suggested that the hormone (hCG-Au15nm) and the receptor (labeled with LHR38-Au5nm) have similar routes of endocytosis, both of them being degraded in lysosomes. Studies of the reappearance of LHR38-Au5nm on the surface of the cells and the use of monensin indicated that only a very small proportion of the receptor molecules were recycled to the cell surface. The distribution and the intracellular pathways of LH receptors are very similar in Leydig cells and transfected L-cells. This opens the possibility of using the latter to study, by in vitro mutagenesis, the molecular mechanisms involved in the cellular traffic of LH receptors.     

Phosphorylation of transfected wild type and mutated progesterone receptors

An expression vector encoding wild type or mutated forms of the rabbit progesterone receptor was transfected into COS-7 cells and phosphorylation was studied by incubation with 32Pi followed by specific immunoprecipitation. The features of phosphorylation of the wild type receptor were identical to those previously observed in uterine cells: there was a basal level of phosphorylation which was increased approximately 7-fold by incubation with the hormone. The hyperphosphorylated receptor had decreased electrophoretic mobility ("upshift"). These experiments thus showed that the presence of the receptor specific kinase is not restricted to the target cells. Cleavage of the receptor by hydroxylamine and cyanogen bromide, and use of receptor mutants deleted in the N-terminal region, showed the absence of any detectable phosphorylation downstream from amino acid 520 (thus in the DNA and steroid binding domains). The majority of the phosphorylation sites were localized between amino acids 166 and 520. This localization was similar for basal and hormone-induced phosphorylation. DNA binding and hormone-induced hyperphosphorylation were not directly related, since deletion of the first zinc finger provided a hyperphosphorylated receptor. We showed that the constitutive receptor (totally deleted in the steroid binding region) exhibited only a low basal level of phosphorylation, and antagonist RU 486-receptor complexes were found to be hyperphosphorylated, leading us to conclude that the active form of the receptor was not the hyperphosphorylated one. Moreover receptor down regulation and hormone-induced receptor hyperphosphorylation were two independent phenomena. Basal phosphorylation was observed for both cytoplasmic and nuclear mutants, whereas nuclear localization was necessary but not sufficient for hyperphosphorylation. Finally, the second finger region and the hormone binding domain, which are necessary for receptor hyperphosphorylation, may be involved in the hormonally induced increased affinity of the receptor toward its kinase.     

The area moment of inertia of the tibia: A risk factor for stress fractures

In a prospective study of stress fractures among Israeli infantry recruits, the area moment of inertia of the tibia was found to have a statistically significant correlation with the incidence of tibial, femoral and total stress fractures. Recruits with "low" area moments of inertia of the tibia were found to have higher stress fracture morbidity than those with "high" area moments of inertia. The best correlation was obtained when the area moment of inertia was calculated about the AP axis of bending at a cross-sectional level corresponding to the narrowest tibial width on lateral X-rays, a point which is at the distal quarter of the tibia. This finding indicates that bending forces about the approximate AP axis are an important causal factor for tibial and many other stress fractures. The bone's bending strength, or ability to resist bending moments, as measured by the area moment of inertia, helps determine risk to stress fracture.     

Paul-Bunnell antigen in lymphoma and leukemia spleens.

Lymphoid cells obtained from spleens of patients with lymphomas or leukemias were studied for the presence of heterophile (Paul-Bunnell (P-B)) antigen. A mixed agglutination (MA) test was established utilizing monolayers of cells attached to poly-L-lysine-coated wells of plastic U plates. After incubation of the monolayers with infectious mononeucleosis (IM) sera, indicator cells, sheep, or trypsinized bovine erythrocytes were added. The results were assessed according to sedimentation patterns of the indicator cells on the monolayers. Positive MA reactions were shown to be due to specific binding of P-B antibodies to the corresponding antigens on the spleen cells. Positive results were obtained with 15 of 37 spleens from patients with Hodgkin's disease, 5 of 8 lymphoma spleens, 4 of 15 chronic myelocytic leukemia spleens and 2 of 4 chronic lymphocytic leukemia spleens. Only 2 of 25 spleens from patients with various other diseases and 1 of 26 apparently normal thymus specimens gave positive results. This study confirmed demonstration of P-B antigen in lymphoma and leukemia by means of absorption experiments, which was reported previously.     

Isolation of alpha-subunits of factor F1 from submitochondrial particles and the reconstitution of active ATPase from isolated alpha-subunits and beta-subunits bound to the mitochondrial membrane.

The Makey & Seal [(1976) Biochim. Biophys. Acta 453, 250--256] method of polyacrylamide-gel electrophoresis in buffer containing 6 M-urea was used to determine the distribution of iron between the N-terminal and C-terminal iron-binding sites of transferrin in human serum. In fresh serum the two sites are unequally occupied; there is preferential occupation of the N-terminal site. On incubation of the serum at 37 degrees C the preference of iron for the N-terminal site becomes more marked. On storage of serum at -15 degrees C the iron distribution changes so that there is a marked preference for the C-terminal site. Dialysis of serum against buffer at pH 7.4 also causes iron to be bound much more strongly by the C-terminal than by the N-terminal site. The original preference for the N-terminal site can be resroted to the dialysed serum by addition of the diffusible fraction.     

X-ray analysis of a progesterone-binding protein (uteroglobin): preliminary results

Uteroglobin, which is a progesterone-binding protein of the rabbit uterine secretion, has been crystallized and subjected to X-ray diffraction analysis. Two crystalline forms have been observed: a triclinic one ($\text{P1, Z = 2, a = 36.36 (4)}\text{A}\text{?}\text{, b = 37.40 (4)}\text{A}\text{?}\text{, c = 53.28 (4)}\text{A}\text{?}\text{, α = 104.6 (1) °, β = 97.0 (1) °, γ = 111.3(1) °}$); and an orthorhombic one for which the cell is C-centred with $\text{a = 50.86 (5)}\text{A}\text{?}\text{, b = 52.22 (5)}\text{A}\text{?}\text{, c = 47.28 (5)}\text{A}\text{?}$, space group C222₁, Z = 4. Three isomorphous derivatives have been obtained. The crystals appear suitable for detailed study of the three-dimensional structure of the protein.