Abstract
The hr-t gene of polyoma virus encodes both the small and middle T (tumor) antigens and exerts pleiotropic effects on cells. By mutating the 3' splice site for middle T mRNA, we have constructed a virus mutant, Py808A, which fails to express middle T but encodes normal small and large T proteins. The mutant failed to induce morphological transformation or growth in soft agar, but did stimulate postconfluent growth of normal cells. Cells infected by Py808A became fully agglutinable by lectins while retaining normal actin cable architecture and normal levels of extracellular fibronectin. These properties of Py808A demonstrated the separability of structural changes at the cell surface from those in the cytoskeleton and extracellular matrix, parameters which have heretofore been linked in the action of the hr-t and other viral oncogenes.