Abstract
Cells dedicate significant energy to build proteins often organized in multiprotein assemblies with tightly regulated stoichiometries. As genes encoding subunits assembling in a multisubunit complex are dispersed in the genome of eukaryotes, it is unclear how these protein complexes assemble. Here, we show that mammalian nuclear transcription complexes (TFIID, TREX-2 and SAGA) composed of a large number of subunits, but lacking precise architectural details are built co-translationally. We demonstrate that dimerization domains and their positions in the interacting subunits determine the co-translational assembly pathway (simultaneous or sequential). The lack of co-translational interaction can lead to degradation of the partner protein. Thus, protein synthesis and complex assembly are linked in building mammalian multisubunit complexes, suggesting that co-translational assembly is a general principle in mammalian cells to avoid non-specific interactions and protein aggregation. These findings will also advance structural biology by defining endogenous co-translational building blocks in the architecture of multisubunit complexes.
Highlights
Cells dedicate significant energy to build proteins often organized in multiprotein assemblies with tightly regulated stoichiometries
Endogenous anti-TAF10 RNA IPs (RIPs)-room temperature (RT)-qPCR from polysome extracts prepared from mouse embryonic stem cells gave nearly identical results, which emphasises the generality of the co-translational pathway for assembly of the mammalian TAF8-TAF10 heterodimer (Fig. 1e; Supplementary Fig. 1)
We found mRNAs coding for known TATA-binding protein (TBP)-interacting proteins: BRF1 coding for a factor important for Pol III transcription[26], BTAF1 coding for a B-TFIID subunit[27], as well as TAF1, whose enrichment on the microarray was somewhat weaker (Fig. 7a)
Summary
Cells dedicate significant energy to build proteins often organized in multiprotein assemblies with tightly regulated stoichiometries. Protein synthesis and complex assembly are linked in building mammalian multisubunit complexes, suggesting that co-translational assembly is a general principle in mammalian cells to avoid non-specific interactions and protein aggregation. Appropriate translation-based mechanisms may exist in the cell to regulate the interactions between specific subunits in order to avoid incorrect non-specific interactions or subunit aggregations in the absence of the correct partner. It is not well understood how functional subunit interactions are regulated in eukaryotic cells. In mammalian cells the TFIID GTF nucleates the assembly of the Pol II preinitiation complex on all protein-coding gene promoters [refs 10,11 and references therein]. Containing TAFs become soluble when co-expressed with their corresponding specific interaction partner[14], suggesting that individual HFD-containing TAFs aggregate without their specific partners
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