Viral cyclins

Cyclins are regulatory subunits of the cyclin-dependent protein kinases (CDKs). Members of this serine-threonine kinase family regulate the progression of cells through the division cycle. Until some years ago, cyclins were presumed to be encoded exclusively by eukaryotic cells. However, sequencing in 1996 of a simian herpesvirus, the herpesvirus saimiri, uncovered an open reading frame with sequence similarity to cellular cyclins. What at the time was a surprise for virologists and cell biologists alike, has become an accepted occurrence now. Eight different cyclin-encoding viruses have been described to date. One of them is the recently discovered human herpesvirus 8 (KSHV) suspected to cause Kaposi's sarcoma and certain B cell-lymphoproliferations in man. The significance of virus-encoded cyclins in the viral life cycle is currently unclear. However, the link between specific cellular cyclins and cancer suggests that virus-encoded cyclins could be involved in oncogenic events associated with these cyclin-encoding viruses.