Intracellular Targeting of Walleye Dermal Sarcoma Virus Orf A (rv-Cyclin)

Abstract

Walleye dermal sarcoma virus (WDSV) induces tumors and allows or possibly directs tumor regression. WDSV encodes a putative cyclin homologue, Orf A, and six variant Orf A transcripts have been identified. Northern analysis indicated that a 3.3-kb transcript, encoding full-length Orf A, is the predominant transcript in developing, but not regressing, tumors. Three Orf A proteins, one full-length and two amino-truncated forms, were expressed in mammalian and piscine cells, and their intracellular locations were determined. The full-length form was nuclear and concentrated in interchromatin granule clusters, defined by colocalization with SC-35. The amino-truncated forms were cytoplasmic. Fusion of amino-terminal portions of Orf A to a heterologous protein demonstrated that residues 1-112 were necessary for nuclear localization. Mutation of aa K80 and/or E110 disrupted nuclear localization, suggesting a mechanism similar to that of cellular A- and D-type cyclins for its nuclear import.

Comparative Pathogenesis of Epsilonretroviruses

“…Recently, it was suggested that only amino-truncated forms of the WDSV cyclin protein are produced in regressing tumors (61,67). The data in these reports show, in agreement with earlier observations (41,42,61), that an mRNA encoding the full-length WDSV cyclin protein is the predominant orf-A transcript in developing tumors.…”

“…The data in these reports show, in agreement with earlier observations (41,42,61), that an mRNA encoding the full-length WDSV cyclin protein is the predominant orf-A transcript in developing tumors. In contrast with earlier reports, the full-length orf-A mRNA was not detected in regressing tumors when a probe homologous to the region of the mRNA encoding the amino terminus of the WDSV cyclin was used; see below (67). Alternatively spliced mRNAs encoding aminotruncated forms of the WEHV1 or WEHV2 rv-cyclins have not been detected in regressing lesions (42) and therefore do not appear to be used by the WEHVs during tumor development and regression.…”

“…These data suggest that the WDSV cyclin, analogous to the accessory genes of other complex oncogenic retroviruses, may regulate the transcription of cellular genes that deregulate cell proliferation and promote cancer (1). It has been proposed that the full-length WDSV cyclin represses viral transcription through Cdk8 in developing tumors and may affect the transcription of cellular genes to promote cell proliferation (67,68). It was also suggested that amino-truncated forms of the WDSV cyclin cannot enter the nucleus, thereby releasing repression of viral transcription in regressing tumors.…”

“…Recently, it was reported that a WDSV amino-truncated Orf-A protein (the product of alternatively spliced mRNAs produced in regressing tumors), lacking its nuclear localization signal, did not enter the nucleus. These data led to the hypothesis that tumor development may be linked to the synthesis of the full-length rv-cyclin (containing the nuclear localization signal) and that regression may be linked to the synthesis of amino-truncated forms of the WDSV cyclin (67).…”

Walleye Dermal Sarcoma Virus Cyclin Interacts with Components of the Mediator Complex and the RNA Polymerase II Holoenzyme

“…Only A1 contains the entire orfa reading frame. During tumor development, only transcript A1 and a transcript containing orfb can be detected by Northern analysis (37,38). The OrfA protein, expressed in cell culture, can inhibit the activity of the WDSV promoter (47), and it is localized in the nucleus in interchromatin granule clusters (IGCs) (38).…”