Mitochondrially tethered Mmm1 can function as a sole lipid transporter at ER-mitochondria contacts

Yeast mitochondria receive the majority of their lipids from the ER via the heterotetrameric ERMES lipid transport complex. This complex is thought to establish a lipid-transporting bridge of fixed composition spanning the space between both organelles. Intriguingly, however, some of the lipid-transporting components of the complex can be replaced by an artificial ER-mitochondria tether without lipid transport activity, questioning ERMES' relevance in lipid transport. Here, we show that Mmm1, one of the four ERMES subunits, alone is sufficient to support ERMES function when it is artificially tethered to mitochondria, provided its lipid-binding domain is intact. Combined with our previous finding that the absence of Mdm12 and Mdm34 can be rescued by the presence of Mmm1 and the artificial tethering protein ChiMERA, our results suggest that Mmm1 can act as the sole lipid transporter at the ER-mitochondrial contact sites, provided that Mdm10 is present, even in the absence of the other two subunits. Thus, our work reconciles ERMES' importance in lipid transport with the fact that the lipid transport activity of some of its components is not strictly necessary for function.