Most eukaryotic telomeres contain a repeating motif with stretches of guanine residues that form a 3′-terminal overhang extending beyond the telomeric duplex region. The telomeric repeat of hypotrichous ciliates, d(T₄G₄), forms a 16-nucleotide 3′-overhang. Such sequences can adopt parallel-stranded as well as antiparallel-stranded quadruplex conformationsin vitro. Although it has been proposed that guanine-quadruplex conformations may have important cellular roles including telomere function, recombination, and transcription, evidence for the existence of this DNA structurein vivohas been elusive to date. We have generated high-affinity single-chain antibody fragment (scFv) probes for the guanine-quadruplex formed by theStylonychiatelomeric repeat, by ribosome display from the Human Combinatorial Antibody Library. Of the scFvs selected, one (Sty3) had an affinity ofK_d= 125 pM for the parallel-stranded guanine-quadruplex and could discriminate with at least 1,000-fold specificity between parallel or antiparallel quadruplex conformations formed by the same sequence motif. A second scFv (Sty49) bound both the parallel and antiparallel quadruplex with similar (K_d= 3–5 nM) affinity. Indirect immunofluorescence studies show that Sty49 reacts specifically with the macronucleus but not the micronucleus ofStylonychia lemnae. The replication band, the region where replication and telomere elongation take place, was also not stained, suggesting that the guanine-quadruplex is resolved during replication. Our results provide experimental evidence that the telomeres ofStylonychiamacronuclei adoptin vivoa guanine-quadruplex structure, indicating that this structure may have an important role for telomere functioning.

Locking down the SARS-CoV-2 spike: Many efforts to develop therapies against severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) are focused on the spike (S) protein trimer that binds to the host receptor. Structures of trimeric S protein show its receptor-binding domain in either an up or a down conformation. Toelzer et al. produced SARS-CoV-2 S in insect cells and determined the structure by cryo–electron microscopy. In their dataset, the closed form was predominant and was stabilized by binding linoleic acid, an essential fatty acid. A similar binding pocket appears to be present in previous highly pathogenic coronaviruses, and past studies suggested links between viral infection and fatty acid metabolism. The pocket could be exploited to develop inhibitors that trap S protein in the closed conformation. Science , this issue p. 725