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).