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The 72K immediate early (IE) 1 protein of human cytomegalovirus and the 68K protein, also encoded by the IE1 gene, were detected by immunoelectron microscopy using monoclonal antibodies specific for the 68K or 72K proteins. Early after infection, both proteins were localized to the nucleus, but with different localization patterns. Late after infection, both of the proteins decreased markedly in the nucleus, in which nucleocapsids appeared. Simultaneously, the 68K protein became diffusely distributed in the cytoplasm and the plasma membrane, whereas the 72K protein was distributed in the nuclear envelope and the plasma membrane. Both proteins were also observed in the coating membrane of the extracellular dense bodies.

Numerous discrete lesions, which we have termed blebs, appeared in the nucleus of BHK cells 10 to 15 h after infection with herpes simplex virus type 1 (HSV-1). They were formed in the inner portion of the nuclear envelope by the apposition of two thickened lamellae overlying a vacuole. As demonstrated by electron microscopic studies, blebs were regular and associated with the peripheral lamina in the nucleus, averaging 3.5 blebs per micron2. They appeared to be associated with an enrichment of the 155K major capsid protein in the nuclear membrane subfractions as compared with the protein composition of nuclei and plasma membrane fractions. We propose that blebs represent the site of assembly of capsid proteins before DNA insertion and eventual envelopment.