Assembly of eIF3 Mediated by Mutually Dependent Subunit Insertion

Abstract

SUMMARY Eukaryotic initiation factor 3 (eIF3), an essential multi-protein complex involved in translation initiation, is composed of 12 tightly associated subunits in humans. While the overall structure of eIF3 is known, the mechanism of its assembly and structural consequences of dysregulation of eIF3 subunit expression seen in many cancers is largely unknown. Here we show that subunits in eIF3 assemble into eIF3 in an interdependent manner. Assembly of eIF3 is governed primarily by formation of a helical bundle, composed of helices extending C-terminally from PCI-MPN domains in eight subunits. We propose that, while the minimal subcomplex of human-like eIF3 functional for translation initiation in cells consists of subunits a, b, c, f, g, i, and m, numerous other eIF3 subcomplexes exist under circumstances of subunit over- or underexpression. Thus, eIF3 subcomplexes formed or “released” due to dysregulated subunit expression may be determining factors contributing to eIF3-related cancers.

Human eIF3b and eIF3a serve as the nucleation core for the assembly of eIF3 into two interconnected modules: the yeast-like core and the octamer

“…siRNA knockdown of eIF3e resulted in simultaneous, severe downregulation of the protein levels of subunits within the entire right side of the eIF3 octamer body: the right leg subunits eIF3k and eIF3l, as well as the non-octameric eIF3d subunit (Figure 3E, Supplementary Figure S4E, and Table 1). The latter is consistent with recent studies where eIF3e was proposed to connect eIF3d to the rest of the octamer (32,40). All other eIF3 subunits were unaffected by eIF3e downregulation.…”

“…Moreover, they support the previous observation that eIF3e directly connects the d subunit to the eIF3 complex (32,40). Finally, these results indicate that besides the left arm subunit eIF3a—which is shared by both the octamer and the YLC modules—both the eIF3c subunit and to a smaller degree also the eIF3e subunit support the attachment of the left leg subunits (f, h, m) to the YLC module via their attachment to the eIF3a subunit.…”

“…The Yeast-Like Core (YLC) comprising the eIF3 subunits a, b, g and i (defined previously (39)) is depicted and so is the eIF3-associated factor eIF3j with arrows indicating its contacts with other eIF3 subunits. The upper right-hand side arrow indicates the interaction between eIF3e and eIF3d that attaches eIF3d to the rest of eIF3 (7,32,40). …”

“…In strong support of this hypothesis, we previously described a stable subcomplex composed of the a, b, g and i subunits of human eIF3 that closely resembles the minimal budding yeast complex and demonstrated that it performs the basic functions of eIF3 in HeLa and HEK293 cells (39). Moreover, others have reported that a similar complex (even containing the eIF3c subunit [NIP1 in yeast]) can form in vitro at increased levels of ionic strength (7), as well as in Neurospora crassa , a fungus whose eIF3 displays the complex subunit composition observed in mammals (40). In accordance with these observations, we will refer to this human a-b-g-i subcomplex as the Y east- L ike C ore (YLC).…”

Recent Discoveries on the Role of TOR (Target of Rapamycin) Signaling in Translation in Plants

“…The eIF3h C-terminal domain is critical to eIF3h integration within eIF3 , suggesting that the reinitiation function depends on the complete eIF3 complex. Interestingly, the eIF3 noncore subunit h was implicated in assembly of subunits e, d, k, and l into the functional eIF3 complex; however, there are no data on this complex formation in plants (des Georges et al, 2015;Smith et al, 2016).…”