The cytoplasmic polyhedrosis virus (CPV) from the family Reoviridae belongs to a subgroup of "turreted" reoviruses, in which the mRNA capping activity occurs in a pentameric turret. We report a full atomic model of CPV built from a 3D density map obtained using cryoelectron microscopy. The image data for the 3D reconstruction were acquired exclusively from a CCD camera. Our structure shows that the enzymatic domains of the pentameric turret of CPV are topologically conserved and that there are five unique channels connecting the guanylyltransferase and methyltransferase regions. This structural organization reveals how the channels guide nascent mRNA sequentially to guanylyltransferase, 7-Nmethyltransferase, and 2′-O-methyltransferase in the turret, undergoing the highly coordinated mRNA capping activity. Furthermore, by fitting the deduced amino acid sequence of the protein VP5 to 120 large protrusion proteins on the CPV capsid shell, we confirmed that this protrusion protein is encoded by CPV RNA segment 7.V iruses of the family Reoviridae have a segmented dsRNA genome enclosed by single, double, or triple capsid shells. They share a similar transcription mechanism in which the inner capsids (core) remain intact and serve as shelters protecting the transcriptional process from antiviral defense mechanisms inside the cytoplasm of the host cell during replication of their dsRNA genomes. Despite the absence of significant sequence homology among different genera of the Reoviridae, the innermost capsid shells, which are formed by two conformers of the one protein, of all members of the Reoviridae and the majority of dsRNA viruses share common functions (1).The "turreted" reoviruses have been defined as a subgroup of the Reoviridae due to their similar protein composition and capsid architecture. For this subgroup of reoviruses, a pentameric turret formed by five copies of turret proteins on fivefold vertex of the innermost shell functions in the catalysis of mRNA 5′ cap synthesis (2-5). Cryo-EM or crystal structures are available for four turreted virus genera of the family Reoviridae: the Cypovirus (6), Orthoreovirus (2), Oryzavirus (7), and Aquareovirus (8, 9) genera. These viruses have a segmented genome consisting of 10-12 linear segments of dsRNA and their hosts include vertebrates (orthoreoviruses and aquareovriuses), invertebrates (cypoviruses), and plants (oryzaviruses).The cytoplasmic polyhedrosis virus (CPV) belongs to the genus Cypovirus and has a dsRNA genome of 10 segments. It is unique among dsRNA viruses in having a single capsid layer (10), which corresponds to the core of orthoreoviruses and functions as a stable mRNA synthesis machine in the cytoplasm of host cells that transcribes mRNAs from the segmented double-stranded RNA templates (11). CPV virions are embedded in polyhedra capable of surviving dehydration, freezing, and chemical treatments that would denature most proteins (12). The polyhedra dissolve in the alkaline midgut of their hosts and release infectious virions during the infection ...