STAS domain structure and function

Abstract

Pendrin shares with nearly all SLC26/SulP anion transporters a carboxy-terminal cytoplasmic segment organized around a Sulfate Transporter and Anti-Sigma factor antagonist (STAS) domain. STAS domains of divergent amino acid sequence exhibit a conserved fold of 4 β strands interspersed among 5 α helices. The first STAS domain proteins studied were single-domain anti-sigma factor antagonists (anti-anti-σ). These anti-anti-σ indirectly stimulate bacterial RNA polymerase by inactivating inhibitory anti-σ kinases, liberating σ factors to direct specific transcription of target genes or operons. Some STAS domains are nucleotide-binding phosphoproteins or nucleotidases. Others are interaction/transduction modules within multidomain sensors of light, oxygen and other gasotransmitters, cyclic nucleotides, inositol phosphates, and G proteins. Additional multidomain STAS protein sequences suggest functions in sensing, metabolism, or transport of nutrients such as sugars, amino acids, lipids, anions, vitamins, or hydrocarbons. Still other multidomain STAS polypeptides include histidine and serine/threonine kinase domains and ligand-activated transcription factor domains. SulP/SLC26 STAS domains and adjacent sequences interact with other transporters, cytoskeletal scaffolds, and with enzymes metabolizing transported anion substrates, forming putative metabolons. STAS domains are central to membrane targeting of many SulP/SLC26 anion transporters, and STAS domain mutations are associated with at least three human recessive diseases. This review summarizes STAS domain structure and function.

Fig. 1

A. X-ray crystal structure of the complex of B. subtilis SpoIIAB anti-ρ homodimer kinase (comprising protomers AB1 (purple) and AB2 (magenta), with the a^F domain of holo-sigma factor 0^F superposed with the complex of SpoIIAB homodimer and two SpoIIAA anti-anti-ρ monomers (gray and green). Nucleotides bound to each SpoIIAB protomer are shown in green stick and active site Mg²⁺ as green balls. Reproduced from [9]. B. SpoIIAB catalytic cycle. Residues important for binding and dissociation are shown in (1): AB1 protomer of SpoIIAB (blue) is targeted by SpoIIAA (orange), as its docking surface (R20 in particular) is more accessible than in AB2 (green). (2) SpoIIAA binds to initial sites on SpoIIAB1 (E104, I112). (3). Bound SpoIIAA D23 interacts with SpoIIAB1 R20, leading to steric clash between SpoIIAA E21 and o^F D148. (4) The steric clash promotes dissociation of o^F from ADP-bound SpoIIAB. SpoIIAA then adopts a conformation that allows S58 phosphorylation (yellow circle changes to red) by SpoIIAA kinase. (5) Phospho-SpoIIAA (yellow) dissociates from ADP-bound SpoIIAB. (6) Unphosphorylated SpoIIAA can bind to SpoIIAB, forming an inhibitory complex that by blocking o^F binding maintains o^F in its active conformation. Reproduced from [13].

Fig. 2

A. The cr^B regulatory pathway of B. subtilis. A. The 1.5 MB stressosome, an ordered 1.5 megadalton complex made up of multiple copies of STAS proteins RsbR and RsbS, serves to sequester kinase RsbT in normal conditions. Under stress, RsbT phosphorylates both STAS proteins, resulting in its release from the stressosome to bind and activate RsbU phosphatase. The RsbT/RsbU copmplex-mediated dephosphorylation of anti-anti-ρ factor STAS protein RsbV allows it to bind anti-ρ factor RsbW, liberating ρ^B from its inactivating complex with RsbW, and allowing activation of RNA polymerase. B. Cryo-transmission electron microscopy-derived molecular envelope of the RbsR/RbsS stressosome at 8Å resolution, viewed down one of its 2-fold axes of symmetry (central black ellipse). Dotted lines mark the other two symmetry axes. Modified from [16].

Fig. 3

A. B. subtilis YtvA electrostatic surface image from LOV domain crystal structure and from STAS domain (modeled on crystal structure of B. subtilis SpoIIAA). Blue light is absorbed by the flavin mononucleotide chromophore of the LOV domain, triggering local conformational change that is believed to be transmitted through the agency of the J linker to the STAS domain, which binds BODIPY-GTP. Yellow residues have been implicated by mutagenesis as important or required for phototransduction. Modified from [21]. B. Light-induced conformational change of YtvA as imagined from holoprotein structure. Modified from [26].

Fig. 4

A. Schematic of M tuberculosis Rv1739c SulP polypeptide, showing its putatively cytoplasmic N-terminal segment, the transmembrane domain with 12 putative transmembrane oc-helices [7, 74], and the cytoplasmic C-terminal STAS domain. B. Secondary structure of the Rv1739c STAS domain deduced from the three dimensional solution structure, aligned above the STAS domain's 124 amino acid residues (aa 437-560 in the holoprotein). Modified from [30].

Fig. 5

A. M. tuberculosis Rv1739c STAS domain backbone structure (blue) with red-highlighted amino acid residues that showed significant chemical shift perturbation (CSP) in the presence of GDP. Side chains of residues W50 and T64 are in cyan. Figure drawn in VMD [74]. B. Rv1739c STAS residues showing GDP-induced CSP mapped onto average STAS structure. Colors denote STAS backbone ¹⁵N-1H relaxation parameters. CSP residues in red show increased R₂ and J(0) along with decreased NOE and J(<&_N) values, indicating high flexibility likely due to increased chemical exchange. CSP residues in yellow deviated from the red pattern in one or two relaxation parameters. CSP residues in blue are those for which at least two relaxation parameters were unavailable due to experimental limitations. W50 and T64 are shown in green. Figure drawn in PyMOL (www.pymol.org). Modified from [30] and from Sharma et al. (manuscript in revision).

Fig. 6

A. Crystal structure of STAS domain from E. coli SulP protein YchM (red) in complex with acyl carrier protein (ACP, cyan), as described by [31]. Shown in yellow is the malonyl-4'-phosphopantheine prosthetic group covalently linked through the phosphate to the hydroxyl group of conserved S36 of ACP. The structure (PDB:3NY7) was rendered in PyMOL. B. Experimental evidence supports the existence of numerous YchM-interacting proteins, grouped here by function (reproduced from [31]). C. Schematic model of YchM as a bicarbonate transporter delivering substrate to STAS domain-bound ACP for sequential transfer to multiple components of the fatty acid biosynthesis pathway assembled around the STAS-ACP complex (reproduced from [31]).

Fig. 7

A. Sulfate assimilation pathways in plants. Reproduced from [38]. B.-D. Model of proposed regulatory function of A. thaliana Sultr1;2 (modified from [41]) in high extracellular [SO₄²⁻] conditions (B. and C.) and in low extracellular [SO₄²⁻] conditions (D.). See text for details.

Fig. 8

STAS domain amino acid sequences from the indicated human and rat SLC26 polypeptides, bacterial SulP proteins Rv1739c and YchM, and bacterial anti-anti-ρ pro teins SpoIIAA and TM1442. The anti-anti-ρ proteins are presented in their full lengths. Amino acid numbe ring is from UniProtKB (www.uniprot.org). The underlined DSSG motif of SpoIIAA includes its phosphorylated residue S58. Gray block indicates the intervening sequence (IVS) present in mammalian SLC26 STAS domains but absent from SulP STAS domains and from anti-anti- ρ proteins. SLC26 STAS amino acid residues in bold highlight human disease- associated missense mutations (in red), termination mutations (in yellow), and frameshifts mutations (in blue). Note that among SLC26 STAS domains, only in pendrin have mutations of the IVS been reported. Asterisks under sequences mark positions of complete sequence conservation. Alignment was generated by ClustalW2 (http://www.ebi.ac.uk/Tools/msa/clustalw2/).

Fig. 9

Backbone structure of the human pendrin STAS domain encompassing aa 515-734 (excluding the intervening sequence (IVS) region of aa 566-653 between helix a1 and strand β3), as modeled on the structure of rat prestin (PDB ID 3LLO; [10]). The missing portions correspond to regions absent from the rat prestin structure. Red residues are sites of disease-associated missense mutations; yellow residues are sites of disease-associated termination mutations. Blue oval marks nominal strand β5 (2 aa in length) which is absent from the structure generated by PyMOL, but present in that generated by MOLMOL (not shown; [75]).