Morphological changes of the insect cells in the baculovirus system as a function of v-myb and c-myb inserts expression and topographic localization of v-Myb and c-Myb proteins.

Structural changes of insect cells Spodoptera frugiperda in the baculovirus expression system after expression of v-myb oncogene and c-myb protooncogene inserts were studied by means of electron microscopy. Expression of v-myb gene insert was accompanied by extensive changes in cell structure, when compared with those of the noninfected and wild-type virus-infected cells. Enormous increase in nuclear content was apparent within 48 hrs after infection, along with changes in nucleolus appearance. Large ring-shaped nucleoli, compact nucleoli and nucleoli with nucleolonemas were detected together with dense nucleolus of normal appearance and small nucleolar structures localized in the nuclear periphery. The cytoplasm practically disappeared 72 hrs after infection. Morphological changes of insect cells expressing the c-myb gene were significantly less distinct, but more frequent unraveling of nucleoli was observed. Both v-Myb and c-Myb proteins were localized in the nucleus of infected cells as was revealed by fluorescence microscopy and electron microscopy. c-Myb marker decorated distinctly the ring-shaped area of nucleolus with some less intensive labelling of the inner part of nucleolus and proximal area on nuclear membrane. v-Myb protein revealed predominantly more compact and homogeneous distribution inside the nucleolus but a small proportion of it was also detected outside the nucleolus in the nuclear compartment. The data obtained on insect cells suggest that Myb proteins may participate also in the processes in which the nucleolus plays a role.