Specificity of the GET pathway for TA protein insertion in Arabidopsis thaliana

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The Guided Entry of Tail-anchored proteins (GET) pathway is viewed as textbook example of TA protein insertion into the ER membrane. The cytosolic ATPase ScGET3 shuttles nascent TA proteins to the ER receptors ScGET1/2 for membrane insertion. While biochemical and structural analyses have established this seemingly compelling model, physiological and genetic evidence for the mechanism is scarce. In yeast, loss of GET function is dispensable and only a limited number of TA proteins have so far been tested to depend on GET for membrane insertion, leaving the question unresolved which alternative pathway(s) is(are) likely responsible for posttranslational insertion of most TA proteins. ScGET3 also acts as a molecular chaperone in protein quality control, binding to misfolded TA proteins and either reverting these to an insertion competent state or targeting them for degradation.We have identified and characterised the main orthologues of a putative GET pathway in plants and uncovered that GET loss-of-function lines in Arabidopsis show shorter root hairs, while otherwise growing normally. While this defect can at least in part be attributed to reduced abundance of an important plasma membrane SNARE (SYP123), lack of a more pronounced phenotype and identification of TA proteins that do not bind to AtGET orthologues suggest existence of alternative insertion pathways. Moreover, aberrant expression of the cytosolic AtGET3a in the Atget1 line leads to a range of severe phenotypes from seedling lethality to reduced growth and fertility. We aim to understand the specificity of AtGET3a in targeting TA proteins to the ER and what, if any, alternative cellular functions does it execute. Also, we seek to determine the key players in the ER membrane for TA protein insertion and identify protein(s) that engage in an alternative insertion pathway.