A systematic microscopy screen uncovers the insertion requirement of yeast mitochondrial tail-anchored proteins

Tail-anchored (TA) proteins are a group of proteins sharing a similar structure: a single C terminal transmembrane domain (TMD). It has been shown that TA protein insertion into the mitochondrial outer membrane (MOM) is independent of the known mitochondrial import components, however no dedicated insertion machinery has been identified. To decipher the molecular mechanisms guiding MOM TA protein insertion, we performed a systematic microscopy screen, whereby we visualized the localization of a model GFP-tagged MOM TA protein (Gem1) on the background of all yeast mutants (in both essential and non-essential genes). We could find no mutant in which GFP-Gem1 insertion into MOM was completely blocked. However, we found that in ∆spf1 cells, GFP-Gem1 and another MOM TA protein were partially localized to the endoplasmic reticulum (ER). Spf1 is an ER ATPase with unknown function, however we show that in ∆spf1 cells mitochondrial and ER membranes become similarly ergosterol poor. Alongside our previous findings that ergosterol content affects the rates of TA protein insertion into mitochondria, we suggest that the inherent differences in membrane composition between various organelle membranes are sufficient to determine membrane-integration specificity of MOM TA proteins in a living cell.