Structural Basis for Noncanonical Substrate Recognition of Cofilin/ADF Proteins by LIM Kinases.

Cofilin/actin-depolymerizing factor (ADF) proteins are critical nodes that relay signals from protein kinase cascades to the actin cytoskeleton, in particular through site-specific phosphorylation at residue Ser3. This is important for regulation of the roles of cofilin in severing and stabilizing actin filaments. Consequently, cofilin/ADF Ser3 phosphorylation is tightly controlled as an almost exclusive substrate for LIM kinases. Here we determine the LIMK1:cofilin-1 co-crystal structure. We find an interface that is distinct from canonical kinase-substrate interactions. We validate this previously unobserved mechanism for high-fidelity kinase-substrate recognition by in vitro kinase assays, examination of cofilin phosphorylation in mammalian cells, and functional analysis in S. cerevisiae. The interface is conserved across all LIM kinases. Remarkably, we also observe both pre- and postphosphotransfer states in the same crystal lattice. This study therefore provides a molecular understanding of how kinase-substrate recognition acts as a gatekeeper to regulate actin cytoskeletal dynamics.

Structure and function of the polymerase core of TRAMP, a RNA surveillance complex.

The Trf4p/Air2p/Mtr4p polyadenylation (TRAMP) complex recognizes aberrant RNAs in Saccharomyces cerevisiae and targets them for degradation. A TRAMP subcomplex consisting of a noncanonical poly(A) RNA polymerase in the Pol ss superfamily of nucleotidyl transferases, Trf4p, and a zinc knuckle protein, Air2p, mediates initial substrate recognition. Trf4p and related eukaryotic poly(A) and poly(U) polymerases differ from other characterized enzymes in the Pol ss superfamily both in sequence and in the lack of recognizable nucleic acid binding motifs. Here we report, at 2.7-A resolution, the structure of Trf4p in complex with a fragment of Air2p comprising two zinc knuckle motifs. Trf4p consists of a catalytic and central domain similar in fold to those of other noncanonical Pol beta RNA polymerases, and the two zinc knuckle motifs of Air2p interact with the Trf4p central domain. The interaction surface on Trf4p is highly conserved across eukaryotes, providing evidence that the Trf4p/Air2p complex is conserved in higher eukaryotes as well as in yeast and that the TRAMP complex may also function in RNA surveillance in higher eukaryotes. We show that Air2p, and in particular sequences encompassing a zinc knuckle motif near its N terminus, modulate Trf4p activity, and we present data supporting a role for this zinc knuckle in RNA binding. Finally, we show that the RNA 3' end plays a role in substrate recognition.

Real-time dynamic single-molecule protein sequencing on an integrated semiconductor device.

Studies of the proteome would benefit greatly from methods to directly sequence and digitally quantify proteins and detect posttranslational modifications with single-molecule sensitivity. Here, we demonstrate single-molecule protein sequencing using a dynamic approach in which single peptides are probed in real time by a mixture of dye-labeled N-terminal amino acid recognizers and simultaneously cleaved by aminopeptidases. We annotate amino acids and identify the peptide sequence by measuring fluorescence intensity, lifetime, and binding kinetics on an integrated semiconductor chip. Our results demonstrate the kinetic principles that allow recognizers to identify multiple amino acids in an information-rich manner that enables discrimination of single amino acid substitutions and posttranslational modifications. With further development, we anticipate that this approach will offer a sensitive, scalable, and accessible platform for single-molecule proteomic studies and applications.

A single active-site region for a group II intron.

Despite the biological importance of self-splicing group II introns, little is known about their structural organization. Synthetic incorporation of site-specific photo-cross-linkers within catalytic domains resulted in functional distance constraints that, when combined with known tertiary interactions, provide a three-dimensional view of the active intron architecture. All functionalities important for both steps of splicing are proximal before the first step, suggestive of a single active-site region for group II intron catalysis.

The receptor for branch-site docking within a group II intron active site.

The distinguishing feature of group II introns, and the property that links them with spliceosomal catalysis, is their ability to undergo splicing through branching. In this reaction, the 2'-hydroxyl group of a specific adenosine within intron domain 6 serves as the nucleophile for attack on the 5' splice site. We know less about branching than any other feature of group II intron catalysis, largely because the receptor structure for activating the branch site is unknown. Here, we identify the intronic region that binds the branch site of a group IIB intron. Located in domain 1, close to receptors for intron domain 5 and both splice sites, we demonstrate that the branch-site receptor is a functional element required for transesterification. Furthermore, we show that crosslinked branch sites can carry out both steps of splicing, suggesting that the conformational state of the intron core is set early and that it persists throughout the entire splicing process.

Properties of the human erythrocyte glucose transport protein are determined by cellular context.

Human erythrocyte hexose transfer is mediated by the glucose transport protein GLUT1 and is characterized by a complexity that is unexplained by available hypotheses for carrier-mediated sugar transport [Cloherty, E. K., Heard, K. S., and Carruthers, A. (1996) Biochemistry 35, 10411-10421]. The study presented here examines the possibility that the operational properties of GLUT1 are determined by host cell environment. A glucose transport-null strain of Saccharomyces cerevisiae (RE700A) was transfected with the p426 GPD yeast expression vector containing DNA encoding the wild-type human glucose transport protein (GLUT1), mutant GLUT1 (GLUT1(338)(-)(A3)), or carboxy-terminal hemagglutinin-polyHis-tagged GLUT1 (GLUT1-HA-H6). GLUT1 and GLUT1-HA-H6 are expressed at the yeast cell membrane and restore 2-deoxy-d-glucose, 3-O-methylglucose, and d-glucose transport capacity to RE700A. GLUT1-HA-H6 confers GLUT1-specific sugar transport characteristics to transfected RE700A, including inhibition by cytochalasin B and high-affinity transport of the nonmetabolized sugar 3-O-methylglucose. GLUT1(338)(-)(A3), a catalytically inactive GLUT1 mutant, is expressed but fails to restore RE700A sugar uptake capacity or growth on glucose. In contrast to transport in human red cells, K(m(app)) for 2-deoxy-d-glucose uptake equals K(i(app)) for 2-deoxy-d-glucose inhibition of 3-O-methylglucose uptake. Unlike transport in human red cells or transport in human embryonic kidney cells transfected with GLUT1-HA-H6, unidirectional sugar uptake in RE700A-GLUT1-HA-H6 is not inhibited by reductant and is not stimulated by intracellular sugar. Net uptake of subsaturating 3-O-methylglucose by RE700A-GLUT1-HA-H6 is a simple, first-order process. These findings support the hypothesis that red cell sugar transport complexity is host cell-specific.

Molecular determinants of sugar transport regulation by ATP.

Intracellular ATP inhibits human erythrocyte net sugar transport by binding cooperatively to the glucose transport protein (GluT1). ATP binding produces altered transporter affinity for substrate and promotes substrate occlusion within a post-translocation vestibule formed by GluT1 cytosolic domains. The accompanying paper (Cloherty, E. K., Levine, K. B., Graybill, C., and Carruthers, A. (2002) Biochemistry 41, 12639-12651) demonstrates that reduced intracellular pH promotes high-affinity ATP binding to GluT1 but inhibits ATP-modulation of GluT1-mediated sugar transport. The present study explores the role of GluT1 residues 326-343 (a proposed GluT1 ATP-binding site subdomain) in GluT1 ATP binding by using alanine scanning mutagenesis. Cos-7 and HEK cells were transfected with a cDNA encoding full-length human GluT1 terminating in a carboxyl-terminal hemagglutinin (HA)-His6 epitope. The transporter (GluT1.HA.H6) is expressed at the surface of both cell-types and is catalytically active. In HEK cells, both parental GluT1- and GluT1.HA.H6-mediated sugar transport are acutely sensitive to cellular metabolic inhibition. Isolated, detergent-solubilized GluT1.HA.H6 is photolabeled by [gamma-32P]-azidoATP in an ATP-protectable manner. Alanine substitution of E329 or G332/R333/R334 enhances GluT1.HA.H6 [gamma-32P]azidoATP photoincorporation but blocks acute modulation of net sugar transport by cellular metabolic inhibition. These actions resemble those of reduced pH on ATP binding to and modulation of red cell GluT1. It is proposed that cooperative nucleotide binding to GluT1 and nucleotide modulation of GluT1-mediated sugar transport are regulated by a proton-sensitive saltbridge (Glu329-Arg333/334).