Table 1.
Roles and partners of bacterial and human PNPase.
Fig 1.
Domains and structure of PNPase.
(A) Domain organization of PNPase, using the C. crescentus PNPase crystal structure as a reference [44]. The complete PNPase trimer viewed from the (B) top and (C) side, with domains colored the same as in (A). (D) A cut-away view of the RNA-bound trimer interior of the PNPase trimer bound to an RNA substrate (orange). Ball and stick structures depict nitrogen atoms as blue and oxygen atoms as red. Inset panels illustrate (i) the interactions between the GSGG loop of each KH domain with the RNA backbone, (ii) base stacking interactions between phenylalanine residues of the FFRR loops and several RNA bases, and (iii) the active site residues that bind phosphate and the metal cofactor. PNPase, polynucleotide phosphorylase.
Fig 2.
The numerous roles performed by PNPase.
Schematic showing the functions performed by PNPase in bacteria and mitochondria. (A) In E. coli and other gram-negative bacteria, PNPase functions as part of the RNA degradosome in bulk mRNA decay but also operates independently of this machine to process tRNAs and rRNAs, add polynucleotide tails to RNAs, and modulate sRNA stability. PNPase binds Hfq-bound sRNAs but only degrades unbound sRNAs, which impacts both positive and negative sRNA-mediated gene regulation in E. coli. In Deinococcus radiodurans, PNPase forms a complex with Rsr mediated via Y-RNAs that degrades misfolded rRNAs. (B) The locations and functions of hPNPase are controversial, but under normal cellular conditions, hPNPase is mainly localized to the IMS where it is able to facilitate translocation of 5S rRNA, RNase P RNA, and possibly RNase MRP RNA into the mitochondrial matrix. Within the mitochondrial matrix, hPNPase degrades mitochondrial RNA and plays a role in mitochondrial DNA maintenance. Release into the cytoplasm upon overexpression or permeabilization of the mitochondrial outer membrane during apoptosis leads to the decay of mRNAs and polyadenylated noncoding RNAs. hfq, host factor for phage Qβ; hPNPase, human PNPase; IMS, inner membrane space; MRP, mitochondrial RNA processing; mtRNA, mitochondrial RNA; PNPase, polynucleotide phosphorylase; Rsr, Ro sixty-related protein; sRNA, small regulatory RNA; SUV3, suppressor of Var 1, 3.