Putative Fusion-Associated Small Transmembrane (FAST) Proteins Encoded by Viruses of Pistolviridae, Order Ghabrivirales, Identified from In Silico Analyses

Fusion-associated small transmembrane (FAST) proteins are viral nonstructural proteins known to be encoded by specific members of the Spinareoviridae, specifically within the Aquareovirus and Orthoreovirus genera. These proteins specialize in mediating cell–cell fusion, leading to syncytia. Unlike enveloped viruses, naked viruses do not rely on fusion proteins for cell entry; however, such proteins may facilitate viral spread between cells. Although not essential for virus replication, FAST proteins have been shown to enhance viral replication, particularly during the early stages of infection. More recently, proteins with characteristics resembling FAST proteins have been identified in a broader range of viruses, including several rotavirus species within the family Sedoreoviridae, and, unexpectedly, in some enveloped viruses within the Coronaviridae family. Here, we present protein sequence analyses suggesting that viruses of the recently established virus family Pistolviridae (order Ghabrivirales) also encode proteins with similarity to FAST proteins. Pistolviruses are small double-stranded RNA viruses that infect piscine species, and were initially referred to as “toti-like” viruses due to genomic similarities with members of the former Totiviridae, which infect single-celled organisms. The putative FAST proteins of the pistolviruses may be expressed either from small, distinct open reading frames or suggested to be produced as cleavage products derived from polyproteins.