A working model for condensate RNA-binding proteins as matchmakers for protein complex assembly

Most cellular processes are carried out by protein complexes, but it is still largely unknown how the subunits of lowly expressed complexes find each other in the crowded cellular environment. Here, we will describe a working model where RNA-binding proteins in cytoplasmic condensates act as matchmakers between their bound proteins (called protein targets) and newly translated proteins of their RNA targets to promote their assembly into complexes. Different RNA-binding proteins act as scaffolds for various cytoplasmic condensates with several of them supporting translation. mRNAs and proteins are recruited into the cytoplasmic condensates through binding to specific domains in the RNA-binding proteins. Scaffold RNA-binding proteins have a high valency. In our model, they use homotypic interactions to assemble condensates and they use heterotypic interactions to recruit protein targets into the condensates. We propose that unoccupied binding sites in the scaffold RNA-binding proteins transiently retain recruited and newly translated proteins in the condensates, thus promoting their assembly into complexes. Taken together, we propose that lowly expressed subunits of protein complexes combine information in their mRNAs and proteins to colocalize in the cytoplasm. The efficiency of protein complex assembly is increased by transient entrapment accomplished by multivalent RNA-binding proteins within cytoplasmic condensates.

Automated summary

Most cellular processes are carried out by protein complexes. The subunits of lowly expressed complexes can find each other through the help of RNA-binding proteins in cytoplasmic condensates. These proteins act as matchmakers between their bound proteins and newly translated proteins to promote the assembly of the complexes. Different RNA-binding proteins act as scaffolds for various cytoplasmic condensates and support translation. mRNA and proteins are recruited into the cytoplasmic condensates through binding to specific domains in the RNA-binding proteins. The scaffold RNA-binding proteins have a high valency and use homotypic interactions to assemble the condensates. They also use heterotypic interactions to recruit protein targets. The unoccupied binding sites in the scaffold RNA-binding proteins retain recruited and newly translated proteins in the condensates, promoting their assembly into complexes. Lowly expressed subunits of protein complexes combine information in their mRNAs and proteins to colocalize in the cytoplasm. The efficiency of protein complex assembly is increased by transient entrapment accomplished by multivalent RNA-binding proteins within cytoplasmic condensates.