The Role of Changing Loop Conformations in Streptavidin Versions Engineered for High-affinity Binding of the Strep-tag II Peptide.

The affinity system based on the artificial peptide ligand Strep-tag® II and engineered tetrameric streptavidin, known as Strep-Tactin®, offers attractive applications for the study of recombinant proteins, from detection and purification to functional immobilization. To further improve binding of the Strep-tag II to streptavidin we have subjected two protruding loops that shape its ligand pocket for the peptide - instead of D-biotin recognized by the natural protein - to iterative random mutagenesis. Sequence analyses of hits from functional screening assays revealed several unexpected structural motifs, such as a disulfide bridge at the base of one loop, replacement of the crucial residue Trp120 by Gly and a two-residue deletion in the second loop. The mutant m1-9 (dubbed Strep-Tactin XT) showed strongly enhanced affinity towards the Strep-tag II, which was further boosted in case of the bivalent Twin-Strep-tag®. Four representative streptavidin mutants were crystallized in complex with the Strep-tag II peptide and their X-ray structures were solved at high resolutions. In addition, the crystal structure of the complex between Strep-Tactin XT and the Twin-Strep-tag was elucidated, indicating a bivalent mode of binding and explaining the experimentally observed avidity effect. Our study illustrates the structural plasticity of streptavidin as a scaffold for ligand binding and reveals interaction modes that would have been difficult to predict. As result, Strep-Tactin XT offers a convenient reagent for the kinetically stable immobilization of recombinant proteins fused with the Twin-Strep-tag. The possibility of reversibly dissociating such complexes simply with D-biotin as a competing ligand enables functional studies in protein science as well as cell biology.

Chronic pulmonary aspergillosis in a tertiary care centre in Spain: A retrospective, observational study.

The aim of this study was to describe the characteristics of patients with chronic pulmonary aspergillosis (CPA) in a tertiary care centre in Spain. Retrospective cohort study of all patients diagnosed with CPA between January 2010 and December 2015. The patients were identified through the Microbiology Registry. Demographic, clinical, laboratory, radiological, microbiological and clinical data were recorded. Patients were followed up for 12 months. Fifty-three patients were included; median age was 61.5 years. Forty-seven had a lung condition, 25 suffered from COPD, 19 an active malignancy, 10 had previous pulmonary tuberculosis and 9 lung interstitial disease. Twenty-eight patients presented with chronic cavitary pulmonary form (CCPA) and 20 with subacute invasive aspergillosis (SAIA). Species identified were A fumigatus (34), A niger (5), A terreus (4) and A flavus (3). All-cause 1-year mortality was 56%. Predictors of mortality were cancer history (OR, 9.5; 95% CI, 2.54-35.51; P < 0.01) and SAIA (OR, 5.49; 95% CI, 1.49-19.82; P < 0.01). Previous pulmonary tuberculosis, surgery for the treatment of CPA and CCPA were found to be associated with lower mortality (OR, 0.05; 95% CI, <0.01-0.47; P < 0.01; OR, 0.16; 95% CI, 0.03-0.88; P = 0.035 and OR 0.2, 95% CI, 0.01-0.67; P = 0.01, respectively). This is the first study providing an overview of the features of CPA in patients from Spain. CCPA was the most frequent form of CPA and A fumigatus the most frequently isolated species. Patients with cancer history and SAIA had a worse prognosis.

Molecular Identification and Susceptibility Testing of Molds Isolated in a Prospective Surveillance of Triazole Resistance in Spain (FILPOP2 Study).

Antifungal resistance is increasing by the emergence of intrinsically resistant species and by the development of secondary resistance in susceptible species. A previous study performed in Spain revealed levels of azole resistance in molds of between 10 and 12.7%, but secondary resistance in Aspergillus fumigatus was not detected. We used itraconazole (ITZ)-supplemented medium to select resistant strains. A total of 500 plates supplemented with 2 mg/liter of ITZ were sent to 10 Spanish tertiary hospitals, and molecular identification and antifungal susceptibility testing were performed. In addition, the cyp51A gene in those A. fumigatus strains showing azole resistance was sequenced. A total of 493 isolates were included in the study. Sixteen strains were isolated from patients with an infection classified as proven, 104 were isolated from patients with an infection classified as probable, and 373 were isolated from patients with an infection classified as colonization. Aspergillus was the most frequent genus isolated, at 80.3%, followed by Scedosporium-Lomentospora (7.9%), Penicillium-Talaromyces (4.5%), Fusarium (2.6%), and the order Mucorales (1%). Antifungal resistance was detected in Scedosporium-Lomentospora species, Fusarium, Talaromyces, and Mucorales Three strains of A. fumigatus sensu stricto were resistant to azoles; two of them harbored the TR34+L98H mechanism of resistance, and the other one had no mutations in cyp51A The level of azole resistance in A. fumigatus remains low, but cryptic species represent over 10% of the isolates and have a broader but overall higher range of antifungal resistance.

Development of the Twin-Strep-tag® and its application for purification of recombinant proteins from cell culture supernatants.

Short peptide affinity tags have become indispensable in protein research. They cannot only be used for affinity purification but also downstream for detection and assay of an arbitrary fused recombinant protein without the need for any prior knowledge of its biochemical properties. Strep-tag®II is particularly popular for providing recombinant proteins at high purity and functionality by using physiological conditions within a rapid one-step protocol. The affinity receptor for Strep-tag®II is affinity engineered streptavidin, named Strep-Tactin®. Strep-tag®II binds to the biotin binding pocket enabling mild competitive elution with biotin derivatives, preferably desthiobiotin, for repeated use of the Strep-Tactin® affinity resins. Fast binding and dissociation kinetics allow comparatively high flow rates throughout column chromatography including elution. Fast dissociation kinetics may be, however, limiting for using Strep-tag®II for direct purification of target proteins from large volumes of diluted extracts like mammalian cell culture supernatants or in assay formats requiring extended washing like ELISA. For this reason, binding characteristics were improved by development of the Twin-Strep-tag® consisting of two Strep-tag®II moieties connected by a short linker. The resulting avidity effect, i.e., the combined synergistic binding of two Strep-tag®II moieties to tetrameric Strep-Tactin®, reduces the off-rate for more steady binding under non-competitive conditions. The addition of a competitor, however, reverses the synergistic avidity effect and, hence, efficient elution capability is preserved. In fact, the Twin-Strep-tag® features all beneficial properties of Strep-tag®II, including efficient elution under gentle competitive conditions, but, due to its higher affinity, additionally enables a more universal use in applications requiring stable binding.