Differential detergent treatment allows immunofluorescent localization of the Newcastle disease virus matrix protein within the nucleus of infected cells.

Paramyxoviruses are cytoplasmic viruses and presumably do not require any nuclear function for their replication. However, recent studies using monoclonal antibodies directed against the Newcastle disease virus matrix (M) protein have found a large portion of the M protein apparently associated with the nucleus of infected cells. Whether the M protein is associated with the cytoplasmic surface of the nucleus or whether the M protein is actually located within the nucleus has not been clearly determined. To examine this question, conditions for selectively permeabilizing the cytoplasmic membrane were sought. After treating fixed cells with a low concentration (0.02%) of the nonionic detergent Triton X-100, the cytoplasmic antigen vimentin was stained with a monoclonal antibody, but nuclear antigens were not. Apparently, 0.02% Triton permeabilizes the plasma membrane while leaving the nuclear membrane intact. Under these conditions, monoclonal antibodies directed against the NDV phosphoprotein and hemagglutinin/neuraminidase glycoprotein stained infected cells, but a monoclonal antibody to the M protein did not. The inability of the anti-M monoclonal antibody to stain the nucleus, even though the outer nuclear membrane is accessible under these conditions, indicates that the M protein is not associated with the outer membrane of the nucleus. The nuclei of infected cells treated with a higher concentration (0.05%) of Triton X-100 were stained both with antibodies to nuclear antigens and with the anti-M monoclonal antibody.