ABCE (OABP) and ABCF (GCN20)
The ABCE and ABCF subfamilies consist of gene products with no TM domain and are not involved in membrane transport functions. They have only ATP-binding domains and no TM domain. The ABCE subfamily has a single member – the oligo-adenylate-binding protein (OABP). This molecule recognizes oligo-adenylate and is produced in response to infection by certain viruses. For example, it interacts with HIV-1 proteins Vif and Gag and the HIV-2 protein GAG. (Dooher & Lingappa, 2004; Zimmerman et al., 2002).
In the ABCF gene family, each gene contains a pair of NBDs. The best-characterized member is the S. cerevisiae GCN20 gene product that mediates the activation of the eIF-2a kinase (Marton, Vazquez de Aldana, Qiu, Chakraburtty, & Hinnebusch, 1997). The human homolog is named ABCF1 and is associated with the ribosome. It appears to have a similar functional role (Tyzack, Wang, Belsham, & Proud, 2000). ABCF1 was also identified as a new retinal pigment epithelium (RPE) phagocytotic ligand by functional screening assays, where it extrinsically promoted phagocytosis of shed photoreceptor cells by the RPE. This function is essential in the visual cycle because RPE cells are specialized phagocytes that maintain retinal homeostasis and prevent retinal degeneration (Guo et al., 2015). Additional roles of ABCF1 include regulating innate immune response and its role as a risk gene for autoimmune pancreatitis and arthritis. Abcf1 expression also plays a role in the development of mouse embryos, and its expression in adult animals correlates with actively proliferating and differentiating cell types (Wilcox et al., 2017).