ZFC3H1 prevents RNA trafficking into nuclear speckles through condensation

Controlling proper RNA pool for nuclear export is important for accurate gene expression. ZFC3H1 is a key controller that not only facilitates nuclear exosomal degradation, but also retains its bound polyadenylated RNAs in the nucleus upon exosome inactivation. However, how ZFC3H1 retains RNAs and how its roles in RNA retention and degradation are related remain largely unclear. Here, we found that upon degradation inhibition, ZFC3H1 forms nuclear condensates to prevent RNA trafficking to nuclear speckles (NSs) where many RNAs gain export competence. Systematic mapping of ZFC3H1 revealed that it utilizes distinct domains for condensation and RNA degradation. Interestingly, ZFC3H1 condensation activity is required for preventing RNA trafficking to NSs, but not for RNA degradation. Considering that no apparent ZFC3H1 condensates are formed in normal cells, our study suggests that nuclear RNA degradation and retention are two independent mechanisms with different preference for controlling proper export RNA pool—degradation is preferred in normal cells, and condensation retention is activated upon degradation inhibition.

Single-molecule measurements reveal that PARP1 condenses DNA by loop stabilization

Poly(ADP-ribose) polymerase 1 (PARP1) is an abundant nuclear enzyme that plays important roles in DNA repair, chromatin organization and transcription regulation. Although binding and activation of PARP1 by DNA damage sites has been extensively studied, little is known about how PARP1 binds to long stretches of undamaged DNA and how it could shape chromatin architecture. Here, using single-molecule techniques, we show that PARP1 binds and condenses undamaged, kilobase-length DNA subject to sub-piconewton mechanical forces. Stepwise decondensation at high force and DNA braiding experiments show that the condensation activity is due to the stabilization of DNA loops by PARP1. PARP inhibitors do not affect the level of condensation of undamaged DNA but act to block condensation reversal for damaged DNA in the presence of NAD+. Our findings suggest a mechanism for PARP1 in the organization of chromatin structure.

The Influence of Silicateins on the Shape and Crystalline Habit of Silica Carbonate Biomorphs of Alkaline Earth Metals (Ca, Ba, Sr)

This contribution presents the effect of two ortholog enzymes from marine sponges called silicateins on the formation of silica carbonate biomorphs of alkaline metals (Ca, Ba, Sr). In vivo, these enzymes participate in the polymerization of silica. Silicateins from Tethya aurantia and Suberitis domuncula were produced recombinantly and presented different degrees of activity, as evidenced by their ability to cleave silyl ether-like bonds in a model compound. Biomorphs are typically inorganic structures that show characteristic shapes resembling those of biological structures such as helices, leaves, flowers, disks or spheres. Irrespective of the concentration or the enzyme used, the presence of silicateins inhibited the formation of classic morphologies of biomorphs, albeit to different extents. Thus, not only the silica condensation activity of the enzyme but also its ability to bind silica compounds is implicated in the inhibition process. The largest effect was observed for the strontium and barium biomorphs, leading to the formation of spheres similar to those observed in diatoms and Radiolaria rather than the classical non-symmetrical forms. Characterization of the samples using Raman spectroscopy showed that silicatein did not affect the crystalline structure of the alkaline earth metal carbonate but did modify the crystalline habit.