This family includes eukaryotic translation initiation factor 6 (eIF6) as well as presumed archaeal homologues.The assembly of 80S ribosomes requires joining of the 40S and 60S subunits, which is triggered by the formation of an initiation complex on the 40S subunit. This event is rate-limiting for translation, and depends on external stimuli and the status of the cell. Eukaryotic translation initiation factor 6 (eIF6) binds specifically to the free 60S ribosomal subunit and prevents its association with the 40S ribosomal subunit ribosomes []. Furthermore, eIF6 interacts in the cytoplasm with RACK1, a receptor for activated protein kinase C (PKC). RACK1 is a major component of translating ribosomes, which harbour significant amounts of PKC. Loading 60S subunits with eIF6 caused a dose-dependent translational block and impairment of 80S formation, which are reversed by expression of RACK1 and stimulation of PKC in vivo and in vitro. PKC stimulation leads to eIF6 phosphorylation and its release, promoting 80S subunit formation. RACK1 provides a physical and functional link between PKC signalling and ribosome activation [, , ].All members of this family have a conserved pentameric fold referred to as a beta/alpha propeller. The eukaryotic IF6 members have a moderately conserved C-terminal extension which is not required for ribosomal binding, and may have an alternative function [].
Shwachman-Bodian-Diamond syndrome (SDS) is an autosomal disorder with pancreatic, skeletal and bone marrow deficiencies and a predisposition to hematological dysfunction. The syndrome is associated with mutations in the SBDS gene, which encodes a protein that is required for biogenesis of the 60S ribosomal subunit and activation of the ribosome []. Maturation of the ribosomal 60S subunit requires SBDS and the GTPase EFL1, which release the anti-association factor eIF6 from the surface of the ribosomal subunit 60S []. In yeast, SBDS is known as Sdo1, which with the EF-2-like GTPase Ria1, may trigger the GTP-dependent release of Tif6 from 60S pre-ribosomes in the cytoplasm, thereby activating them [].This entry also includes Sdo1/SBDS related proteins, including Rtc3 from budding yeasts. The function of Rtc3 is not clear.
