Size selection

In RNA interference (RNAi), short interfering (si)RNAs silence gene expression in a sequence-specific manner, and plants use RNAi to defend against RNA viruses. However, many of these viruses have a counter-defence mechanism. For example, the 19-kDa protein (p19) of tombusviruses can suppress RNA silencing, and has been shown to bind specifically to siRNAs. But how does p19 recognize siRNAs? Two papers now provide insights. Patel and co-workers (reporting in Nature) and Tanaka Hall and colleagues (reporting in Cell) both used X-ray crystallography to study a complex between p19 and a 21-nucleotide (19-base-pair duplex) siRNA.

These studies showed that a p19 homodimer binds to siRNA, and a concave β-sheet of this homodimer spans all 19 base pairs along one face of the siRNA duplex. This interaction is sequence independent (all the interactions are directed towards the sugar–phosphate backbone in p19–siRNA), but is dependent on the size of the siRNA duplex. Each p19 monomer contains a pair of tryptophan residues that are positioned such that, in the homodimer, they stack over the terminal base pairs, and so measure and 'cap' the ends of the siRNA duplex. Using biochemical and in vivo assays, Tanaka Hall and colleagues verified the importance of these tryptophan residues in suppressing RNA silencing and, in addition, defined the siRNA characteristics that are important for recognition by p19. Together, the work by these groups might help us to further understand — and manipulate — RNA-silencing mechanisms in heterologous systems. REFERENCES Ye, K. et al. Recognition of small interfering RNA by a viral suppressor of RNA silencing. Nature 426, 874–878 (2003) Vargason, J. M. et al. Size selective recognition of siRNA by an RNA silencing suppressor. Cell 115, 799–811 (2003)

Helical bundle conversion

Vinculin is a component and regulator of cell–cell (cadherin-mediated) and cell–matrix (integrin–talin-mediated focal adhesions) junctions. In its resting state, its head (Vh) and tail (Vt) domains interact to form a closed confirmation, but Vt can bind to the actin cytoskeleton when Vh binds to α-actinin in cadherin junctions and to talin in focal adhesions. In Nature, Bois and colleagues have now defined the mechanism of vinculin activation by determining the crystal structures of human vinculin in its inactive and talin-activated forms.

In its inactive form, Vh is composed of seven α-helices that are arranged as two four-helix bundles (α4 is long and spans both bundles), and Vh interacts with Vt to form the expected closed structure. On binding talin, marked changes occur in the amino-terminal bundle of Vh — the four-helix bundle completely rearranges to form a five-helix bundle that incorporates a helix from talin. No structural changes occur in the carboxy-terminal helical bundle of Vh, which functions as a scaffold. As both α-actinin and talin can bind Vh and displace Vt, it seems that, by inducing structural changes in Vh, these proteins can activate vinculin in cadherin junctions and focal adhesions, respectively. This work has also established “...helical bundle conversion as a signalling mechanism by which proteins direct cellular responses”. REFERENCES Izard, T. et al. Vinculin activation by talin through helical bundle conversion. Nature 427, 171–175 (2004)