The Human Cytomegalovirus Assembly Compartment: A Masterpiece of Viral Manipulation of Cellular Processes That Facilitates Assembly and Egress

A characteristic feature of human cytomegalovirus (HCMV) infected cells is an enlarged, kidney-shaped nucleus wrapping around a juxtanuclear body (also referred to as a perinuclear body) called the viral cytoplasmic assembly compartment (AC) (Figure 1A). This association of the nucleus and AC is vital for virion assembly and egress. The AC was first described [1,2] while examining the localization of HCMV tegument proteins, which are incorporated into the virion. It was found that they localized to a juxtanuclear compartment that overlapped the endoplasmic reticulum-Golgi intermediate compartment. Detailed studies by Pellet and coworkers [3–5] have shown that the AC results from extensive remodeling of the secretory apparatus. From these data a three-dimensional model was proposed of a cylindrical AC composed of many organelle-specific vesicles (Golgi, trans-Golgi network, and early endosomes), which form nested, cylindrical layers (depicted by the colored circles in Figure 1B). More recently, additional cellular components have been found associated with the AC, including markers of the late endocytic pathway, lysosomal markers, SNARE family members [6,7], and the ER chaperone glucose-regulated protein 78 (GRP78) also known as BiP [8,9]. BiP appears to play a role in AC stability; BiP depletion causes rapid AC disintegration and the cessation of infectious virion formation [9]. In addition, components of the ESCRT (endosomal sorting complex required for transport) machinery may associate with the AC. Since ESCRT controls the incorporation of cargo into intraluminal vesicles of multivesicular bodies, this association suggests that the ESCRT machinery may facilitate formation of the multivesicular AC and virion maturation [4]. Recent evidence shows that the AC also contains mammalian target of rapamycin (mTOR) kinase [10]. During infection HCMV strives to maintain mTOR kinase activity due to its importance in maintaining cellular processes needed by the virus. The data suggest that sequestration of mTOR and its activator Rheb-GTP in the AC helps maintain mTOR activity and protect mTOR from inhibition by cellular stress responses induced during lytic infection [10]. The AC also contains HCMVencoded Fc receptor-like proteins which have an affinity for rabbit immunoglobulin G (IgG) [11], which can cause spurious localization of IgG to the assembly compartment. Thus when studying the AC by immunofluorescence it is best to use nonrabbit antibodies, or block with HCMV-negative human serum, which effectively binds the Fc receptors [9,12]. The purpose of the Fc receptor-like proteins in the AC is unknown. A complete list of known cellular components of the AC, and how their function may be altered during infection, can be found in [4]. This compilation of cellular components in the AC is based on static imaging, thus dynamic partitioning of other cellular proteins is quite likely. Overall, the data show that the HCMV-induced remodeling of the membrane transport apparatus is extensive, altering the organization of secretory organelles in order to facilitate assembly and egress of virions.