HAR1 mediates systemic regulation of symbiotic organ development

Symbiotic root nodules are beneficial to leguminous host plants; however, excessive nodulation damages the host because it interferes with the distribution of nutrients in the plant. To keep a steady balance, the nodulation programme is regulated systemically in leguminous hosts. Leguminous mutants that have lost this ability display a hypernodulating phenotype. Through the use of reciprocal and self-grafting studies using Lotus japonicus hypernodulating mutants, har1 (also known as sym78), we show that the shoot genotype is responsible for the negative regulation of nodule development. A map-based cloning strategy revealed that HAR1 encodes a protein with a relative molecular mass of 108,000, which contains 21 leucine-rich repeats, a single transmembrane domain and serine/threonine kinase domains. The har1 mutant phenotype was rescued by transfection of the HAR1 gene. In a comparison of Arabidopsis receptor-like kinases, HAR1 showed the highest level of similarity with CLAVATA1 (CLV1). CLV1 negatively regulates formation of the shoot and floral meristems through cell-cell communication involving the CLV3 peptide. Identification of hypernodulation genes thus indicates that genes in leguminous plants bearing a close resemblance to CLV1 regulate nodule development systemically, by means of organ-organ communication.     

Radixin deficiency causes conjugated hyperbilirubinemia with loss of Mrp2 from bile canalicular membranes

The ezrin-radixin-moesin (ERM) family of proteins crosslink actin filaments and integral membrane proteins. Radixin (encoded by Rdx) is the dominant ERM protein in the liver of wildtype mice and is concentrated at bile canalicular membranes (BCMs). Here we show that Rdx(-/-) mice are normal at birth, but their serum concentrations of conjugated bilirubin begin to increase gradually around 4 weeks, and they show mild liver injury after 8 weeks. This phenotype is similar to human conjugated hyperbilirubinemia in Dubin-Johnson syndrome, which is caused by mutations in the multidrug resistance protein 2 (MRP2, gene symbol ABCC2), although this syndrome is not associated with overt liver injury. In wildtype mice, Mrp2 concentrates at BCMs to secrete conjugated bilirubin into bile. In the BCMs of Rdx(-/-) mice, Mrp2 is decreased compared with other BCM proteins such as dipeptidyl peptidase IV (CD26) and P-glycoproteins. In vitro binding studies show that radixin associates directly with the carboxy-terminal cytoplasmic domain of human MRP2. These findings indicate that radixin is required for secretion of conjugated bilirubin through its support of Mrp2 localization at BCMs.     

Actomyosin structure in contracting muscle detected by rapid freezing

It is now widely accepted that the ATP-induced active sliding of adjacent thin and thick filaments mediated by myosin heads (cross-bridges) is responsible for muscle contraction. Despite intensive studies, the behaviour of the myosin heads during muscle contraction is still unclear. Recent progress in the rapid freezing electron microscope technique has greatly improved the temporal resolution of the images that can be obtained. Here, we report a new type of actomyosin structure captured by rapid freezing. We have analysed images from thin sections of freeze-substituted rabbit skeletal muscle rapidly frozen during isometric contraction. For comparison, we also studied relaxed and rigor muscles. Our results show that, during isometric contraction, most myosin heads are regularly arrayed along the helix of the actin filaments and that this actomyosin structure appears to be distinct from that observed in rigor muscle.