Circoviruses represent a rapidly expanding group of viruses that infect both vertebrate and invertebrate hosts. Members are responsible for diseases of veterinary and economic importance, including postweaning multisystemic wasting syndrome in pigs, and beak and feather disease (BFD) in birds. These viruses are associated with lymphoid depletion and immunosuppressive conditions in infected animals leading to systemic illness. Circoviruses are small nonenveloped DNA viruses containing a single-stranded circular genome, encoding two major proteins: the capsid-associated protein (Cap), comprising the entirety of the viral capsid, and the replication-associated protein (Rep). Cap is the only protein component of the virion and plays crucial roles throughout the virus replication cycle, including viral attachment, cell entry, genome uncoating, and packaging of newly formed viral particles. Rep mediates recognition of replication origin motifs in the viral genome sequence and is responsible for endonuclease activity enabling nicking of the circular DNA and initiation of rolling-circle replication (RCR). Porcine circovirus 2 (PCV2) was the first circovirus capsid structure to be solved at atomic resolution using X-ray crystallography. The structure revealed an assembly comprising 60 monomeric subunits to form virus-like particles. Each Cap monomer harbors a canonical viral jelly roll domain composed of two, four-stranded antiparallel b-sheets. Crystal structures of two distinct macromolecular assemblies from BFD virus Cap were also resolved at high resolution. In these structures, the exposure of the N-terminal arginine-rich motif, responsible for DNA binding and nuclear localization is reversed. Additional structural investigations have also elucidated a PCV2 type-specific neutralizing epitope, and interaction between the PCV2 capsid and polymers such as heparin. In this review, we provide a snapshot of the structural and functional aspects of circovirus proteins.

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A 6 year old female Spitz dog was admitted to Teaching Veterinary Hospital at Chittagong Veterinary and Animal Sciences University with the history of inappetance and asymmetric lower jaw. Oral inspection revealed a pair of pale enlarged lobulated mass around the third molar tooth of the left mandible. The dog was clinically diagnosed with epulis and successfully cured by surgical excision. Histopathological section of the excised masses revealed characteristic features of acanthomatous ameloblastoma with some atypical lesions. Multifocal areas of ameloblastic islands were found in the dense sheet of proliferating epithelial layer protruding towards the sub epithelial connective tissue stroma. These islands were characterized with irregular epithelial stratification at the basal layer. Besides, presence of ghost cells was the unusual findings for such case. Prominent intercellular bridging and nuclear polymorphism in odontogenic cells were other decisive characters of the lesion. Based on the histomorphological appearance, the gingival tumor was designated as canine acanthomatous ameloblastoma (CAA) with atypical histomorphology.

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