Endoplasmic reticulum: a focal point of Zika virus infection

Mohd Ropidi, Muhammad Izzuddin; Khazali, Ahmad Suhail; Nor Rashid, Nurshamimi; Yusof, Rohana

Next, vesicle packets (VPs) are also commonly observed in ZIKV-infected cells ( Fig. 1 ) [32, [35] [36] [37] . VPs refers to an aggregate of closely packed single-membrane vesicles that are encapsulated within an ER cisterna. Individually, these vesicles measure between 60 and 100 nm in diameter with a narrow channel of approximately 10 nm that connects the vesicle lumen and the cytosol [32, 35] . The radial dimension of these vesicles varies according to the host cell type, thereby hinting possible involvement of cell-type-specific factors in its formation [36] . Modest radial difference was also noted between ZIKV lineages [32] ; however, more morphologically pronounced difference between the ZIKV lineages is observed in the vesicle's shape whereby African strain tend to form ovoid vesicles, whereas the Asian strain's vesicles are generally more spherical [37] . Nevertheless, these differences, especially the latter, remain contentious due to limited experimental measurements and warrants additional investigations across various cell types to support these observations. Despite these differences, the vesicles' pore maintain a comparable diameter of 10 nm regardless of the virus strain and the host cell type, suggesting that a conserved cellular or viral machinery mediates pore formation [32] . ZIKV exploits the ER dynamic characteristic and remodels the ER structure to generate virus-induced structures, vesicles (Ve), vesicle packet (VP), convoluted membrane (CM) and zippered ER (zER), for the benefit of virus replication. Blue and green box depicts schematic representation of host and viral factors involved in vesicle and virus particle (Vi) formation, respectively. ZIKV NS4A utilizes the host reticulon 3.1A (RTN3.1A) to facilitate membrane curvature during vesicle invagination into the ER lumen (blue box). Viral genome replication takes place within this vesicle. Neosynthesized viral RNA genome is released into the cytosol and could undergo either translation, virus particle assembly, or another round of genome replication. Virus particle assembly takes place in apposed ER leaflet. Separate ZIKV NS2A independently recruits viral genome RNA, NS2B-NS3, and unprocessed C-prM-Env complexes, and subsequently congregates at the virus particle assembly site through NS2A oligomerization (green box). Once assembled, NS2B-NS3 proteolytical activity cleaves the recruited C-prM-Env complex to generate individual capsid, prM and Env protein. Cleaved capsid protein interacts with capsid-dense lipid droplets and viral RNA to generate nucleocapsid core followed by Env and prM proteins encapsulation of the nucleocapsid core Invagination of ZIKV-induced vesicles into the ER lumen is mediated through the host reticulon 3.1A (RTN 3.1A) that facilitates ER membrane curvature (Fig. 1 , blue box) [38] . Silencing of RTN3.1A impairs NS4A protein stability and results in significant reduction of virusinduced vesicles and virus replication [38] . Typically, these vesicles contain the virus' replication complex and therefore carries out the virus RNA synthesis or viral replication [32, 36] . Following viral replication, neosynthesized viral genome RNA is released into the cytosol through the vesicle's narrow channel, where it would consequently either undergo another round of genome replication, translation or virion assembly [17, 30] . In the latter case, ZIKV virion assembles and buds into the ER lumen directly apposed the narrow pore of the replication ves