Chromosome-associated spot formation by human cytomegalovirus immediate early 1 (IE1) protein

HCMV is associated with severe disease in immunocompromised patients and is a cause of congenital disease. Already more than 30 years ago, IE1 was described to bind to mitotic chromosomes, however the implications of this finding have yet to be determined. HCMV can infect a wide range of cell types and was also detected in different tumours; however, the majority of molecular studies relating to this virus are performed in fibroblasts or monocytic cells. We examined the localization of HCMV IE1 across multiple cell types, including tumour cell lines, to better understand how the protein behaves in these cellular contexts. IE1 is one of the first HCMV genes expressed after lytic infection of the cell and it was observed to distribute evenly over the chromosomes in so-called "chromosome painting" pattern. In leukemia and glioblastoma cells we detected a novel mitotic localization pattern of IE1 - in chromosome-associated spots (CAS), in addition to "painting the chromosomes", as reported before. We demonstrate that the IE1 core domain mediates CAS localization and that the distribution pattern of IE1 on mitotic chromosomes is influenced by both the cell type being infected and the level of IE1 expression. The novel CAS localization of IE1 suggests a new, possibly cell-type specific function of this protein, distinct from its role in transcriptional regulation.