Distribution of ferritin-conjugated lectins on sialidase-treated membranes of human erythrocytes.

The labeling of sialidase-treated, human erythrocyte membranes with ferritin-conjugates of four plant lectins, concanavalin A, Ricinus communis hemagglutinin, Bauhinia purpurea hemagglutinin and Arachis hypogoea hemagglutinin, is reported. Among these ferritin-conjugated lectins, ferritin-conjugated concanavalin A and ferritin-conjugated R. communis hemagglutinin were found in clusters on the sialidase-treated membranes, whereas ferritin-conjugated B. purpurea hemagglutinin and ferritin-conjugated A. hypogoea hemagglutinin were found in a random distribution on the membranes. Furthermore, when the membranes were labeled with a mixutre of concanavalin A and ferritin-conjugated B. purpurea hemagglutinin, ferritin particles were found in clusters, indicating that the membrane receptors for B. purpurea hemagglutinin were forced to more together with those for concanavalin A. A method for the quantitative estimation of the clustering of ferritin particles on the membranes was also devised and applied to the labeling of sialidase-treated, human erythrocyte membranes with the above four ferritin-conjugated lectins.     

Mutation of glycine 49 to valine in the alpha subunit of GS results in the constitutive elevation of cyclic AMP synthesis

The G-protein GS couples hormone-activated receptors with adenylyl cyclase and stimulates increased cyclic AMP synthesis. Transient expression in COS-1 cells of cDNAs coding for the GS alpha-subunit (alpha S) or alpha S cDNAs having single amino acid mutations Gly49----Val or Gly225----Thr elevated cyclic AMP levels, resulting in the activation of cyclic AMP dependent protein kinase. Stable expression in Chinese hamster ovary cells of alpha S Val49 cDNA resulted in a small constitutive elevation of cyclic AMP that was sufficient to persistently activate cyclic AMP dependent protein kinase activity 1.5-2-fold over basal activity. Stable expression of wild-type alpha S or alpha S Thr225 in Chinese hamster ovary cells was less effective in sustaining elevated cyclic AMP synthesis and kinase activation compared to alpha SVal49.     

Prostaglandin E₂ increases both osteoblastic and osteoclastic activity in the scales and participates in calcium metabolism in goldfish

Using our original in vitro assay system with goldfish scales, we examined the direct effect of prostaglandin E? (PGE?) on osteoclasts and osteoblasts in teleosts. In this assay system, we measured the activity of alkaline phosphatase (ALP) and tartrate-resistant acid phosphatase (TRAP) as respective indicators of each activity in osteoblasts and osteoclasts. ALP activity in scales significantly increased following treatment at high concentration of PGE?(10?? and 10?? M) over 6 hrs of incubation. At 18 hrs of incubation, ALP activity also significantly increased in the PGE? (10?? to 10?? M)-treated scale. In the case of osteoclasts, TRAP activity tended to increase at 6 hrs of incubation, and then significantly increased at 18 hrs of incubation by PGE? (10(-7) to 10?? M) treatment. At 18 hrs of incubation, the mRNA expression of osteoclastic markers (TRAP and cathepsin K) and receptor activator of the NF-κB ligand (RANKL), an activating factor of osteoclasts expressed in osteoblasts, increased in PGE? treated-scales. Thus, PGE? acts on osteoblasts, and then increases the osteoclastic activity in the scales of goldfish as it does in the bone of mammals. In an in vivo experiment, plasma calcium levels and scale TRAP and ALP activities in the PGE?-injencted goldfish increased significantly. We conclude that, in teleosts, PGE? activates both osteoblasts and osteoclasts and participates in calcium metabolism.     

FtsZ from divergent foreign bacteria can function for cell division in Escherichia coli

FtsZs from Mycoplasma pulmonis (MpuFtsZ) and Bacillus subtilis (BsFtsZ) are only 46% and 53% identical in amino acid sequence to FtsZ from Escherichia coli (EcFtsZ). In the present study we show that MpuFtsZ and BsFtsZ can function for cell division in E. coli provided we make two modifications. First, we replaced their C-terminal tails with that from E. coli, giving the foreign FtsZ the binding site for E. coli FtsA and ZipA. Second, we selected for mutations in the E. coli genome that facilitated division by the foreign FtsZs. These suppressor strains arose at a relatively high frequency of 10(-3) to 10(-5), suggesting that they involve loss-of-function mutations in multigene pathways. These pathways may be negative regulators of FtsZ or structural pathways that facilitate division by slightly defective FtsZ. Related suppressor strains were obtained for EcFtsZ containing certain point mutations or insertions of yellow fluorescent protein. The ability of highly divergent FtsZs to function for division in E. coli is consistent with a two-part mechanism. FtsZ assembles the Z ring, and perhaps generates the constriction force, through self interactions; the downstream division proteins remodel the peptidoglycan wall by interacting with each other and the wall. The C-terminal peptide of FtsZ, which binds FtsA, provides the link between FtsZ assembly and peptidoglycan remodeling.     

Myosin Loop 2 Is Involved in the Formation of a Trimeric Complex of Twitchin, Actin, and Myosin

Molluscan smooth muscles exhibit a low energy cost contraction called catch. Catch is regulated by twitchin phosphorylation and dephosphorylation. Recently, we found that the D2 fragment of twitchin containing the D2 site (Ser-4316) and flanking immunoglobulin motifs (TWD2-S) formed a heterotrimeric complex with myosin and with actin in the region that interacts with myosin loop 2 (Funabara, D., Hamamoto, C., Yamamoto, K., Inoue, A., Ueda, M., Osawa, R., Kanoh, S., Hartshorne, D. J., Suzuki, S., and Watabe, S. (2007) J. Exp. Biol. 210, 4399–4410). Here, we show that TWD2-S interacts directly with myosin loop 2 in a phosphorylation-sensitive manner. A synthesized peptide, CAQNKEAETTGTHKKRKSSA, based on the myosin loop 2 sequence (loop 2 peptide), competitively inhibited the formation of the trimeric complex. Isothermal titration calorimetry showed that TWD2-S binds to the loop 2 peptide with a K_a of (2.44 ± 0.09) × 10⁵ m⁻¹ with two binding sites. The twitchin-binding peptide of actin, AGFAGDDAP, which also inhibited formation of the trimeric complex, bound to TWD2-S with a K_a of (5.83 ± 0.05) × 10⁴ m⁻¹ with two binding sites. The affinity of TWD2-S to actin and myosin was slightly decreased with an increase of pH, but this effect could not account for the marked pH dependence of catch in permeabilized fibers. The complex formation also showed a moderate Ca²⁺ sensitivity in that in the presence of Ca²⁺ complex formation was reduced.Molluscan smooth muscles, such as mussel anterior byssus retractor muscle (ABRM)² and adductor muscle, exhibit a low energy cost phase of tension maintenance termed catch. Catch muscle develops active tension following an increase of the intracellular [Ca²⁺] induced by secretion of acetylcholine. Myosin is activated by direct binding of Ca²⁺ to the regulatory myosin light chain and initiates a relative sliding between thick and thin filaments (1). After a decrease of intracellular [Ca²⁺] to resting levels, the catch state is formed where tension is maintained over long periods of time with little energy consumption (2, 3). Catch tension is abolished by secretion of serotonin and an increase of intracellular [cAMP] with the resulting activation of cAMP-dependent protein kinase and phosphorylation of twitchin (4, 5). Twitchin phosphorylation is required for relaxation of the muscle from catch. For this cycle to repeat, dephosphorylation of twitchin is necessary (6). Thus, in this scheme, twitchin is a major regulator of the catch state.Molluscan twitchin is known as a myosin-binding protein belonging to the titin/connectin superfamily. It is a single polypeptide of 530 kDa containing multiple repeats of immunoglobulin (Ig) and fibronectin type 3-like motifs in addition to a single kinase domain homologous to the catalytic domain of myosin light chain kinase of vertebrate smooth muscle (7). There are several possible phosphorylation sites in molluscan twitchin recognized by cAMP-dependent protein kinase, and two, D1 and D2, have been identified. The D1 phosphorylation site (Ser-1075) is in the linker region between the 7th and 8th Ig motifs (numbering from the N terminus). The D2 site (Ser-4316) is in the linker region between the 21st and 22nd Ig motifs. Additional sites are found close to D1, but are thought not to be vital for catch regulation.The molecular mechanisms underlying development and maintenance of the catch state have been controversial for several years. One theory proposes that catch reflected attached frozen or slowly cycling cross-bridges (8, 9). What distinguished the attached cross-bridge from the detached relaxed state is not clear. Also it was suggested that interactions between thick filaments, other than cross-bridges, or between thin and thick filaments are responsible for the catch contraction (10). In either of the latter cases, the cross-bridge (myosin head) was not involved.Recently we found that a twitchin fragment including the D2 phosphorylation site and its flanking Ig motifs (TWD2-S) interacted with myosin and actin in a phosphorylation-sensitive manner, and it was suggested that this trimeric complex contributed to tension maintenance in catch (11). TWD2-S bound to a region of the actin molecule known also to interact with loop 2 of myosin that is involved in the ATP-driven movement of myosin with actin (12). In the present study, we show that the myosin loop 2 binds to TWD2-S using competitive cosedimentation assays and isothermal titration calorimetry (ITC). These techniques were applied to also study in more detail the interactions of the twitchin-binding peptide of actin (identified in the previous study (11)). In addition, the effects of pH and Ca²⁺ on the binding of TWD2-S to myosin and actin were investigated.