a, Yeast cells expressing a fluorescently tagged homo-octameric dipeptidase from E. coli (PDB accession 1POK). The octamer can be viewed as two rings of four subunits each, stacked tail-to-tail. The localization of the wild-type protein is cytosolic and uniform, but a point mutant (E239Y) triggers head-to-head interactions between octamers and their stacking into micrometre-long fibres. b, Assembly is not mediated by interactions between YFP tags. It has been reported that fluorescent proteins can induce protein aggregation through dimerization65. In our work, none of the wild-type proteins aggregated despite their fusion to YFP, probably because we used the variant bearing the A206K mutation disrupting a weak tendency to dimerize. In addition, we performed a control experiment consisting of co-expressing an excess of the dipeptidase (PDB accession 1POK) untagged with YFP, together with a sub-stoichiometric quantity of the YFP-tagged subunit. As a result, most octamers will harbour zero or one YFP tag. In this context, the YFP allows monitoring the assembly but does not participate in multivalent interactions. c, Fluorescence microscopy revealed that fibres were forming in this context and were about tenfold less fluorescent than the case where all subunits were tagged with YFP. Together, these data indicate that YFP is not mediating fibre assembly in vivo.