Ursula Schulze-Gahmen

Structure-based functional inference in structural genomics

The dramatically increasing number of new protein sequences arising from genomics 4 proteomics requires the need for methods to rapidly and reliably infer the molecular and cellular functions of these proteins. One such approach, structural genomics, aims to delineate the total repertoire of protein folds in nature, thereby providing three-dimensional folding patterns for all proteins and to infer molecular functions of the proteins based on the combined information of structures and sequences. The goal of obtaining protein structures on a genomic scale has motivated the development of high throughput technologies and protocols for macromolecular structure determination that have begun to produce structures at a greater rate than previously possible. These new structures have revealed many unexpected functional inferences and evolutionary relationships that were hidden at the sequence level. Here, we present samples of structures determined at Berkeley Structural Genomics Center and collaborators’ laboratories to illustrate how structural information provides and complements sequence information to deduce the functional inferences of proteins with unknown molecular functions.Two of the major premises of structural genomics are to discover a complete repertoire of protein folds in nature and to find molecular functions of the proteins whose functions are not predicted from sequence comparison alone. To achieve these objectives on a genomic scale, new methods, protocols, and technologies need to be developed by multi-institutional collaborations worldwide. As part of this effort, the Protein Structure Initiative has been launched in the United States (PSI; www.nigms.nih.gov/funding/psi.html). Although infrastructure building and technology development are still the main focus of structural genomics programs [1−6], a considerable number of protein structures have already been produced, some of them coming directly out of semi-automated structure determination pipelines [6−10]. The Berkeley Structural Genomics Center (BSGC) has focused on the proteins of Mycoplasma or their homologues from other organisms as its structural genomics targets because of the minimal genome size of the Mycoplasmas as well as their relevance to human and animal pathogenicity (http://www.strgen.org). Here we present several protein examples encompassing a spectrum of functional inferences obtainable from their three-dimensional structures in five situations, where the inferences are new and testable, and are not predictable from protein sequence information alone.

Structural basis for CDK6 activation by a virus-encoded cyclin

Cyclin from herpesvirus saimiri (Vcyclin) preferentially forms complexes with cyclin-dependent kinase 6 (CDK6) from primate host cells. These complexes show higher kinase activity than host cell CDKs in complex with cellular cyclins and are resistant to cyclin-dependent inhibitory proteins (CDKIs). The crystal structure of human CDK6–Vcyclin in an active state was determined to 3.1 Å resolution to better understand the structural basis of CDK6 activation by viral cyclins. The unphosphorylated CDK6 in complex with Vcyclin has many features characteristic of cyclinA-activated, phosphorylated CDK2. There are, however, differences in the conformation at the tip of the T-loop and its interactions with Vcyclin. Residues in the N-terminal extension of Vcyclin wrap around the tip of the CDK6 T-loop and form a short β-sheet with the T-loop backbone. These interactions lead to a 20% larger buried surface in the CDK6–Vcyclin interface than in the CDK2–cyclinA complex and are probably largely responsible for the specificity of Vcyclin for CDK6 and resistance of the complex to inhibition by INK-type CDKIs.

HIV-1 Tat recruits transcription elongation factors dispersed along a flexible AFF4 scaffold

The HIV-1 Tat protein stimulates viral gene expression by recruiting human transcription elongation complexes containing P-TEFb, AFF4, ELL2, and ENL or AF9 to the viral promoter, but the molecular organization of these complexes remains unknown. To establish the overall architecture of the HIV-1 Tat elongation complex, we mapped the binding sites that mediate complex assembly in vitro and in vivo. The AFF4 protein emerges as the central scaffold that recruits other factors through direct interactions with short hydrophobic regions along its structurally disordered axis. Direct binding partners CycT1, ELL2, and ENL or AF9 act as bridging components that link this complex to two major elongation factors, P-TEFb and the PAF complex. The unique scaffolding properties of AFF4 allow dynamic and flexible assembly of multiple elongation factors and connect the components not only to each other but also to a larger network of transcriptional regulators.

AFF1 is a ubiquitous P-TEFb partner to enable Tat extraction of P-TEFb from 7SK snRNP and formation of SECs for HIV transactivation

Significance Transcriptional elongation by RNA polymerase II produces full-length RNA transcripts and plays a general and prominent role in regulating gene expression. The positive transcription elongation factor b (P-TEFb) is one of the most important transcription factors controlling this process. The core P-TEFb, consisting of cyclin-dependent kinase 9 and cyclin T (CycT), exists in a network of complexes that include the 7SK small nuclear ribonucleoprotein particle (7SK snRNP), the super elongation complexes (SECs), and the bromodomain protein 4 (Brd4)–P-TEFb complex. This study identifies AF4/FMR2 family member 1 (AFF1) as a ubiquitous binding partner of core P-TEFb throughout the entire P-TEFb network. By increasing the affinity of the HIV-encoded transactivating (Tat) protein for CycT1, AFF1 is required for Tat’s extraction of P-TEFb from 7SK snRNP and the formation of SECs for maximal HIV transcriptional activation. The positive transcription elongation factor b (P-TEFb) stimulates RNA polymerase elongation by inducing the transition of promoter proximally paused polymerase II into a productively elongating state. P-TEFb itself is regulated by reversible association with various transcription factors/cofactors to form several multisubunit complexes [e.g., the 7SK small nuclear ribonucleoprotein particle (7SK snRNP), the super elongation complexes (SECs), and the bromodomain protein 4 (Brd4)–P-TEFb complex] that constitute a P-TEFb network controlling cellular and HIV transcription. These complexes have been thought to share no components other than the core P-TEFb subunits cyclin-dependent kinase 9 (CDK9) and cyclin T (CycT, T1, T2a, and T2b). Here we show that the AF4/FMR2 family member 1 (AFF1) is bound to CDK9–CycT and is present in all major P-TEFb complexes and that the tripartite CDK9–CycT–AFF1 complex is transferred as a single unit within the P-TEFb network. By increasing the affinity of the HIV-encoded transactivating (Tat) protein for CycT1, AFF1 facilitates Tat’s extraction of P-TEFb from 7SK snRNP and the formation of Tat–SECs for HIV transcription. Our data identify AFF1 as a ubiquitous P-TEFb partner and demonstrate that full Tat transactivation requires the complete SEC.

The AFF4 scaffold binds human P-TEFb adjacent to HIV Tat

Human positive transcription elongation factor b (P-TEFb) phosphorylates RNA polymerase II and regulatory proteins to trigger elongation of many gene transcripts. The HIV-1 Tat protein selectively recruits P-TEFb as part of a super elongation complex (SEC) organized on a flexible AFF1 or AFF4 scaffold. To understand this specificity and determine if scaffold binding alters P-TEFb conformation, we determined the structure of a tripartite complex containing the recognition regions of P-TEFb and AFF4. AFF4 meanders over the surface of the P-TEFb cyclin T1 (CycT1) subunit but makes no stable contacts with the CDK9 kinase subunit. Interface mutations reduced CycT1 binding and AFF4-dependent transcription. AFF4 is positioned to make unexpected direct contacts with HIV Tat, and Tat enhances P-TEFb affinity for AFF4. These studies define the mechanism of scaffold recognition by P-TEFb and reveal an unanticipated intersubunit pocket on the AFF4 SEC that potentially represents a target for therapeutic intervention against HIV/AIDS. DOI: http://dx.doi.org/10.7554/eLife.00327.001

Crystal structure of a hypothetical protein, TM841 of Thermotoga maritima, reveals its function as a fatty acid–binding protein

We determined the three‐dimensional (3D) crystal structure of protein TM841, a protein product from a hypothetical open‐reading frame in the genome of the hyperthermophile bacterium Thermotoga maritima, to 2.0 Å resolution. The protein belongs to a large protein family, DegV or COG1307 of unknown function. The 35 kDa protein consists of two separate domains, with low‐level structural resemblance to domains from other proteins with known 3D structures. These structural homologies, however, provided no clues for the function of TM841. But the electron density maps revealed clear density for a bound fatty‐acid molecule in a pocket between the two protein domains. The structure indicates that TM841 has the molecular function of fatty‐acid binding and may play a role in the cellular functions of fatty acid transport or metabolism. Proteins 2003;50:526–530.

Crystal structure of a viral cyclin, a positive regulator of cyclin-dependent kinase 6

BACKGROUND Cyclin-dependent kinases (CDKs) have a central role in cell-cycle control and are activated by complex formation with positive regulatory proteins called cyclins and by phosphorylation. The overexpression and mutation of cyclins and CDKs has been associated with tumorigenesis and oncogenesis. A virus-encoded cyclin (v-cyclin) from herpesvirus saimiri has been shown to exhibit highest sequence homology to type D cyclins and specifically activates CDK6 of host cells to a very high degree. RESULTS We have determined the first X-ray structure of a v-cyclin to 3.0 A resolution. The structure of the core domains is very similar to those of cyclin A and cyclin H from human cells. To understand the structural basis for the v-cyclin specificity for CDK6 and the insensitivity of the complex to inhibitors of the p21 and INK4 families, a v-cyclin-CDK2 model was built on the basis of the known structures of human cyclin A in complex with CDK2 and the CDK inhibitor p27(Kip1). CONCLUSIONS Although many critical interactions between cyclin A and CDK2 would be conserved in a v-cyclin-CDK2 complex, some appear sterically or electrostatically unfavorable due to shifts in the backbone conformation or sidechain differences and may contribute to v-cyclin selectivity for CDK6. The insensitivity of v-cyclin-CDK6 complexes to inhibitors of the p21 family is probably due to structural changes in v-cyclin that lead to a flatter surface area offering fewer potential contacts with the protein inhibitor. In addition, sequence changes in v-cyclin eliminate hydrogen-bonding partners for atoms of the p27(Kip1) inhibitor. This structure provides the first model for interactions between v-cyclins and host cell-cycle proteins; these interactions may be important for virus survival as well as oncogenic transformation of host cells.

Gene target specificity of the Super Elongation Complex (SEC) family: how HIV-1 Tat employs selected SEC members to activate viral transcription

The AF4/FMR2 proteins AFF1 and AFF4 act as a scaffold to assemble the Super Elongation Complex (SEC) that strongly activates transcriptional elongation of HIV-1 and cellular genes. Although they can dimerize, it is unclear whether the dimers exist and function within a SEC in vivo. Furthermore, it is unknown whether AFF1 and AFF4 function similarly in mediating SEC-dependent activation of diverse genes. Providing answers to these questions, our current study shows that AFF1 and AFF4 reside in separate SECs that display largely distinct gene target specificities. While the AFF1-SEC is more potent in supporting HIV-1 transactivation by the viral Tat protein, the AFF4-SEC is more important for HSP70 induction upon heat shock. The functional difference between AFF1 and AFF4 in Tat-transactivation has been traced to a single amino acid variation between the two proteins, which causes them to enhance the affinity of Tat for P-TEFb, a key SEC component, with different efficiency. Finally, genome-wide analysis confirms that the genes regulated by AFF1-SEC and AFF4-SEC are largely non-overlapping and perform distinct functions. Thus, the SEC represents a family of related complexes that exist to increase the regulatory diversity and gene control options during transactivation of diverse cellular and viral genes.

AFF4 binding to Tat-P-TEFb indirectly stimulates TAR recognition of super elongation complexes at the HIV promoter

Superelongation complexes (SECs) are essential for transcription elongation of many human genes, including the integrated HIV-1 genome. At the HIV-1 promoter, the viral Tat protein binds simultaneously to the nascent TAR RNA and the CycT1 subunit of the P-TEFb kinase in a SEC. To understand the preferential recruitment of SECs by Tat and TAR, we determined the crystal structure of a quaternary complex containing Tat, P-TEFb, and the SEC scaffold, AFF4. Tat and AFF4 fold on the surface of CycT1 and interact directly. Interface mutations in the AFF4 homolog AFF1 reduced Tat–AFF1 affinity in vivo and Tat-dependent transcription from the HIV promoter. AFF4 binding in the presence of Tat partially orders the CycT1 Tat–TAR recognition motif and increases the affinity of Tat-P-TEFb for TAR 30-fold. These studies indicate that AFF4 acts as a two-step filter to increase the selectivity of Tat and TAR for SECs over P-TEFb alone. DOI: http://dx.doi.org/10.7554/eLife.02375.001

Structure of the hypothetical Mycoplasma protein MPN555 suggests a chaperone function.

The crystal structure of the hypothetical protein MPN555 from Mycoplasma pneumoniae (gi|1673958) has been determined to a resolution of 2.8 Angstrom using anomalous diffraction data at the Se-peak wavelength. Structure determination revealed a mostly alpha-helical protein with a three-lobed shape. The three lobes or fingers delineate a central binding groove and additional grooves between lobes 1 and 3 and between lobes 2 and 3. For one of the molecules in the asymmetric unit, the central binding pocket was filled with a peptide from the uncleaved N-terminal affinity tag. The MPN555 structure has structural homology to two bacterial chaperone proteins: SurA and trigger factor from Escherichia coli. The structural data and the homology to other chaperone proteins suggests an involvement in protein folding as a molecular chaperone for MPN555.