viroplasms

Viroplasms are cytoplasmic, membrane-free organelles formed in cells infected by certain viruses, serving as sites of viral replication and assembly. The term is most often used in the study of dsRNA viruses of the Reoviridae family, including rotavirus, but it has broader applicability to viral replication factories observed in other virus groups.

Viroplasms form through interactions between viral nonstructural proteins and viral genomes. In rotavirus-infected cells, proteins such as NSP2 and NSP5 drive the formation of spherical or irregular condensates near the nucleus, which recruit viral RNA segments and additional replication factors. Their formation can involve phase separation, producing dynamic, liquid-like bodies that concentrate viral components while excluding some host proteins.

Functionally, viroplasms concentrate the components needed for RNA synthesis and genome assembly, creating a localized environment with high levels of RNA polymerases, nucleotides, and transcripts. This arrangement can enhance replication efficiency and may shield replicating RNA from host innate immune sensors.

Detection and scope: viroplasms are typically identified by microscopy using antibodies against viral proteins or double-stranded RNA, and they are distinguished from classic membrane-bound organelles by their lack of enclosing membranes. Although well characterized in rotavirus biology, similar replication compartments are observed across various virus families, and the term viroplasm is sometimes used more broadly to refer to such viral replication factories or inclusion bodies.