Avian erythroblastosis virus See avian leukaemia virus

avian flu Avian influenza, bird flu A highly contagious strain of influenza that affects poultry and can be transmitted to humans; having switched species it could become extremely virulent (a similar phenomenon occurred with the 1918 strain of flu, which caused more deaths than the preceding 4 years of war). The H5N1 strain is currently causing considerable concern.

avian leukaemia virus Group of C-type RNA tumour viruses (Oncovirinae) that cause various leukaemias and other tumours in birds. The acute leukaemia viruses, which are replication-defective and require helper viruses, include avian erythroblastosis (AEV), myeloblastosis (AMV) and myelocytomatosis viruses. AEV carries two transforming genes, v-erbA and v-erbB; the cellular homologue of the latter is the structural gene for the epidermal growth factor receptor. AMV carries v-myb and causes a myeloid leukaemia; avian myelocytomatosis virus carries v-myc. The avian lymphatic leukaemia viruses (ALV) are also Retroviridae, but are replication-competent and induce neoplasia only after several months; they often occur in conjunction with replication-defective leukaemia viruses.

avian myeloblastosis virus AMV Retrovirus of the Subfamily Oncornaviridae. Causes myelocytomatosis, osteopetrosis, lymphoid leucosis and nephroblastoma. May be a mixture of viruses.

avidin Biotin-binding protein (68 kDa) from egg white. Binding is so strong as to be effectively irreversible - a diet of raw egg white leads to biotin deficiency.

avidity Strength of binding, usually of a small molecule with multiple binding sites by a larger one; particularly the binding of a complex antigen by an antibody. (Affinity refers to simple receptor-ligand systems.)

avilamycin Oligosaccharide antibiotic that has been used in animal feed for many years. See everninomicins.

avirulent Organism or virus that does not cause infection or disease.

axenic A situation in which only one species is present. Thus an axenic culture is uncontaminated by organisms of other species, an axenic organism does not have commensal organisms in the gut, etc. Some organisms have obligate symbionts and cannot be grown axenically.

axial filaments The central filaments, of which there may be several hundred, of the periplasmic flagella of spirochaetes that rotate within the periplasmic space and cause the whole bacterium to flex like a corkscrew and thus to move. The central filaments are composed of at least three proteins (FlaBl, FlaB2 and FlaB3) that have significant homology with flagellin; the sheath is composed of FlaA protein (~43 kDa).

axil Member of the axin family. Interacts with GSK3 and (-catenin. By enhancing phosphorylation and thus the subsequent degradation of (-catenin, inhibits axis formation in Xenopus embryos.

axin A negative regulator of the Wnt signalling pathway. Binds to APC (adenomatous polyposis coli protein) and to (-catenin. Interacts directly with glycogen synthase kinase-3 (GSK3) and promotes GSK3 phosphorylation of (-catenin, which is then degraded. Axin is encoded by the fused locus in mice that is required for normal vertebrate axis formation. Dvl (dishevelled protein), axin and GSK can form a ternary complex bridged by axin, and Frat1 can be recruited into this complex probably by Dvl. Wnt-1 appears to promote the disintegration of the Fratl-Dvl-GSK-Axin complex, resulting in the dissociation of GSK from axin.

axokinin Axonemal protein (56 kDa) that, when phospho-rylated by a cAMP-dependent protein kinase, reactivates the axoneme.

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