GET complex

Abstract: Deficiencies in mitochondrial import cause the toxic accumulation of non-imported mitochondrial precursor proteins. Numerous fates for non-imported mitochondrial precursors have been identified in budding yeast, including proteasomal destruction, deposition into protein aggregates, and mistargeting to other organelles. Amongst organelles, the ER has emerged as a key destination for a subset of non-imported mitochondrial proteins. However, how ER targeting of various types of mitochondrial proteins is achieved remains incompletely understood. Here, we show that the ER delivery of endogenous mitochondrial transmembrane proteins, especially those belonging to the SLC25A mitochondrial carrier family, is dependent on the guided entry of tail-anchored proteins (GET) complex. Without a functional GET pathway, non-imported mitochondrial proteins destined for the ER are alternatively sequestered into Hsp42-dependent protein foci. Loss of the GET pathway is detrimental to yeast cells experiencing mitochondrial import failure and prevents re-import of mitochondrial proteins from the ER via the ER-SURF pathway. Overall, this study outlines an important role for the GET complex in ER targeting of non-imported mitochondrial carrier proteins.

Abstract: The extensive evolution of intracellular compartmentalization requires highly selective mechanisms for protein targeting to distinct membrane systems. One of the fundamental processes in protein targeting is the insertion of proteins into biological membranes. The efficient and accurate insertion of membrane proteins is an important step for their proper function in different organelles, and any targeting error may lead to mislocalization of these proteins with detrimental cellular effects. Posttranslational insertion is required for a class of tail-anchored (TA) proteins, which are characterized by a transmembrane domain (TMD) near their C terminus, for their correct targeting to the destined membrane (1). In PNAS, Xing et al. uncover a pathway for TA protein insertion into the endoplasmic reticulum (ER) in the model plant Arabidopsis thaliana , which plays an unexpected role in root hair growth (2). The authors identify several key components in the guided entry of tail-anchored protein (GET) complex that has a conserved function in regulating TA protein insertion. However, in contrast to yeast and animals, the core GET system in Arabidopsis involves a distinct GET3 clade, suggesting an ancient evolution of the GET3 paralogs in plants and which may function divergently as plant-specific organelle chaperones. The GET pathway is the most extensively studied system for shuttling TA proteins to distinct organelles in yeast and mammalian cells. Structural and biochemical investigations in yeast have provided insights into the molecular mechanism of the GET complex (1). In the yeast GET pathway, newly synthesized TA proteins are initially recognized by a cytosolic pretargeting complex (PTC), comprising SGT2 and GET4–GET5, and then transferred to the ATPase GET3. GET3 loaded with a TA protein will then bind to its receptor: the ER-localized GET1–GET2 complex, which subsequently promotes the insertion of the TA protein into the ER membrane (Fig. 1 A ). The recognition and insertion processes mediated … [↵][1]1To whom correspondence should be addressed. Email: ljiang{at}cuhk.edu.hk. [1]: #xref-corresp-1-1

Abstract: Deficiencies in mitochondrial import cause the toxic accumulation of non-imported mitochondrial precursor proteins. Numerous fates for non-imported mitochondrial precursors have been identified, including proteasomal destruction, deposition into protein aggregates, and mis-targeting to other organelles. Amongst organelles, the endoplasmic reticulum (ER) has emerged as a key destination for non-imported mitochondrial proteins, but how ER-targeting of these proteins is achieved remains unclear. Here, we show that the guided entry of tail-anchored proteins (GET) complex is required for ER-targeting of endogenous mitochondrial multi-transmembrane proteins. Without a functional GET pathway, non-imported mitochondrial proteins destined for the ER are alternatively sequestered into Hsp42-dependent protein foci. The ER targeting of non-imported mitochondrial proteins by the GET complex prevents cellular toxicity and facilitates re-import of mitochondrial proteins from the ER via the recently identified ER-SURF pathway. Overall, this study outlines an important and unconventional role for the GET complex in mitigating stress associated with non-imported mitochondrial proteins.

Abstract: Tail-anchored (TA) proteins contain a single transmembrane domain (TMD) at the C-terminus. The post-translational insertion of TA proteins into the ER membrane requires the cooperation of the Golgi ER-trafficking (GET) complex, which contains Get1, Get2 and Get3. Get3 is a cytosolic ATPase which can recognize and bind the TMD of the TA proteins. Get1 and Get2 are ER transmembrane proteins which can recruit and form a complex with TA-bound Get3. The GET complex carries out an energy-dependent process that facilitates the insertion of the TA-protein TMD into the ER membrane. In order to investigate the mechanism by which the GET complex functions to promote protein insertion into the ER membrane, yeast Get3 has been crystallized. The crystals diffracted to 2.7 A resolution using a synchrotron X-ray source. The crystals belonged to space group P2(1)2(1)2, with unit-cell parameters a = 220.26, b = 112.95, c = 48.27 A. There is one Get3 dimer in the asymmetric unit, which corresponds to a solvent content of approximately 65%.