Structural complexity of the co-chaperone SGTA: a conserved C-terminal region is implicated in dimerization and substrate quality control

Abstract

BackgroundProtein quality control mechanisms are essential for cell health and involve delivery of proteins to specific cellular compartments for recycling or degradation. In particular, stray hydrophobic proteins are captured in the aqueous cytosol by a co-chaperone, the small glutamine-rich, tetratricopeptide repeat-containing protein alpha (SGTA), which facilitates the correct targeting of tail-anchored membrane proteins, as well as the sorting of membrane and secretory proteins that mislocalize to the cytosol and endoplasmic reticulum-associated degradation. Full-length SGTA has an unusual elongated dimeric structure that has, until now, evaded detailed structural analysis. The C-terminal region of SGTA plays a key role in binding a broad range of hydrophobic substrates, yet in contrast to the well-characterized N-terminal and TPR domains, there is a lack of structural information on the C-terminal domain. In this study, we present new insights into the conformation and organization of distinct domains of SGTA and show that the C-terminal domain possesses a conserved region essential for substrate processing in vivo.ResultsWe show that the C-terminal domain region is characterized by α-helical propensity and an intrinsic ability to dimerize independently of the N-terminal domain. Based on the properties of different regions of SGTA that are revealed using cell biology, NMR, SAXS, Native MS, and EPR, we observe that its C-terminal domain can dimerize in the full-length protein and propose that this reflects a closed conformation of the substrate-binding domain.ConclusionOur results provide novel insights into the structural complexity of SGTA and provide a new basis for mechanistic studies of substrate binding and release at the C-terminal region.Electronic supplementary materialThe online version of this article (10.1186/s12915-018-0542-3) contains supplementary material, which is available to authorized users.

The client-binding domain of the cochaperone Sgt2 has a helical-hand structure that binds a short hydrophobic helix

“…The well-resolved, sharp, but narrowly dispersed chemical shifts of the backbone amide protons in 1H-15N HSQC spectra of Sgt2-C ( Fig. 1D,E), indicate a significant degree of backbone mobility, similar to natively unfolded proteins [46] and consistent with results seen by others [47], further highlighting the lack of stable tertiary structure. [12].…”

SGTA associates with intracellular aggregates in neurodegenerative diseases

“…SGTA has been reported to associate with the hydrophobic domain of membrane proteins through its C-terminal domain [ 27 , 28 ], and to cooperate with their binding partners to maintain cytosolic protein quality control [ 9 ]. In this study, immunohistochemical analysis indicated that SGTA interacts with polyQ aggregates through the C-terminal domain.…”

SGTA associates with intracellular aggregates in neurodegenerative diseases.

“…SGTA has been reported to associate with the hydrophobic domain of membrane proteins through its Cterminal domain, 27,28 and to cooperate with their binding partners to maintain cytosolic protein quality control. 9 In this study, immunohistochemical analysis indicated that SGTA interacts with polyQ aggregates through the C-terminal domain.…”