The contribution of the cytoplasmic retrieval signal of severe acute respiratory syndrome coronavirus to intracellular accumulation of S proteins and incorporation of S protein into virus-like particles

The cytoplasmic tails of some coronavirus (CoV) spike (S) proteins contain an endoplasmic reticulum retrieval signal (ERRS) that can retrieve S proteins from the Golgi to the endoplasmic reticulum (ER); this process is thought to accumulate S proteins at the CoV budding site, the ER-Golgi intermediate compartment (ERGIC), and to facilitate S protein incorporation into virions. However, we showed previously that porcine epidemic diarrhoea CoV S proteins lacking the ERRS were efficiently incorporated into virions, similar to the original virus. Thus, the precise role of the ERRS in virus assembly remains unclear. Here, the roles of the S protein ERRS in severe acute respiratory syndrome CoV (SARS-CoV) intracellular trafficking and S incorporation into virus-like particles (VLPs) are described. Intracellular trafficking and indirect immunofluorescence analysis suggested that when M protein was present, wild-type S protein (wtS) could be retained in the pre- and post-medial Golgi compartments intracellularly and colocalized with M protein in the Golgi. In contrast, mutant S protein lacking the ERRS was distributed throughout the ER and only partially co-localized with M protein. Moreover, the intracellular accumulation of mutant S protein, particularly at the post-medial Golgi compartment, was significantly reduced compared with wtS. A VLP assay suggested that wtS that reached the post-medial compartment could be returned to the ERGIC for subsequent incorporation into VLPs, while mutant S protein could not. These results suggest that the ERRS of SARS-CoV contributes to intracellular S protein accumulation specifically in the post-medial Golgi compartment and to S protein incorporation into VLPs.