ON RIBONUCLEASE ACTIVITY IN RETICULOCYTE RIBOSOMES

Characterization of Novel Ribosome-Associated Endoribonuclease SLFN14 from Rabbit Reticulocytes

Turnover of mRNA is a critical step that allows cells to control gene expression. Endoribonucleases, enzymes cleaving RNA molecules internally, are some of the key components of the degradation process. Here we provide a detailed characterization of novel endoribonuclease SLFN14 purified from rabbit reticulocyte lysate. Schlafen genes encode a family of proteins limited to mammals. Their cellular function is unknown or incompletely understood. In reticulocytes, SLFN14 is strongly overexpressed, represented exclusively by the short form, all tethered to ribosomes, and appears to be one of the major ribosome-associated proteins. SLFN14 binds to ribosomes and ribosomal subunits in the low part of the body and cleaves RNA but preferentially rRNA and ribosome-associated mRNA. This results in the degradation of ribosomal subunits. This process is strictly Mg(2+)- and Mn(2+)-dependent, NTP-independent, and sequence nonspecific. However, in other cell types, SLFN14 is a full-length solely nuclear protein, which lacks ribosomal binding and nuclease activities. Mutational analysis revealed the ribosomal binding site and the aspartate essential for the endonucleolytic activity of protein. Only few endoribonucleases participating in ribosome-mediated processes have been characterized to date. Moreover, none of them are shown to be directly associated with the ribosome. Therefore, our findings expand the general knowledge of endoribonucleases involved in mammalian translation control.

Erythroid cell RNase: activation by urea and localization to the cell membrane

Assays of ribonuclease activity in components of mature and immature mammalian erythroid cells indicate that RNase activity is present both in the membrane-free hemolysate and the washed membranes. Erythroid cell RNase exists in an active and latent form. The majority of total cell RNase activity is in the latent state, and is localized to the erythroid cell membrane. Both total and latent RNase activity decline as the cell matures. The latent RNase is released from its relatively firm attachment to the cell membrane and activated by centrifugation or, optimally, by exposure to 4 M urea. The active sites of membrane-associated RNase are apparently oriented toward the inner side of the cell membrane. The properties of the latent membrane-bound RNase which is activated by urea, including K(m), pH optimum, inhibition of enzyme activity by cations, and response to metabolic inhibitors, do not differ significantly from those of the soluble RNase in the membrane-free hemolysate, suggesting that there is only one type of RNase in the erythroid cell. Binding of Rnase to the erythroid cell membrane stabilized the enzyme against inactivation during incubation at 37 degrees C, and the findings suggest that membrane-bound RNase may play a particular part in degrading ribosomes. The findings indicate that the cell membrane has a major role in RNA metabolism in the maturing mammalian erythroid cell.

RNase activity in erythroid cell lysates

The characteristics of degradation of reticulocyte ribonucleic acid (RNA) and ribosomes were studied in a whole erythroid cell lysate system. The process followed Michaelis-Menten kinetics, and indicated that RNA degradation in the erythroid cell is mediated by an enzyme previously isolated from reticulocyte hemolysates. Erythroid cell RNase activity had a temperature optimum of 50 degrees C, a pH optimum of 7.0, was not energy dependent, was heat labile at physiologic pH, and was inhibited by Mg(++), Ca(++), and exposure to bentonite and deoxycholate. Free sulfhydryl groups were not essential for RNase activity. Of the substrates occurring naturally within the erythroid cell, isolated ribosomal RNA was most susceptible to the action of the enzyme, intact ribosomes least susceptible, and transfer RNA intermediate between them. Natural substrates were degraded completely to nucleotides in cell lysates. Competitive inhibition studies indicate that one enzyme system is capable of degrading both RNA and ribosomes, although the existence of more than one enzyme has not been excluded. Erythroid cell lysates quickly broke down polyribosomes into single ribosomes. The more rapid degradation of ribosomes, as compared with transfer RNA, which occurs in vivo, as opposed to findings in vitro, suggests that there is a special intracellular mechanism responsible for ribosome degradation in the maturing erythroid cell.

Characteristics of RNA degradation in the erythroid cell

The characteristics of ribonucleic acid (RNA) degradation in intact reticulocytes have been investigated. The rate of degradation during in vitro maturation at 37 degrees C is approximately 4% per hr. RNA degradation does not proceed at 0 degrees C, and the rate of degradation is temperature dependent with an optimum at 50 degrees C. The process is not dependent upon glycolysis. Although all types of RNA progressively decrease during in vivo maturation, ribosomes are degraded at a rate greater than that of soluble RNA.RNA within the cell fluxes freely back and forth between the free and membrane-bound forms. The rate of loss of RNA in the membrane-bound compartment, both in the intact cell and in isolated membranes, is approximately threefold that found in the cell as a whole. During maturation the proportion of total cell RNA which is bound to the membrane decreases. The bulk of RNA destroyed is derived from the free compartment, but bound is also destroyed. The cell membrane participates in RNA metabolism and may be a site of RNA degradation in the erythroid cell.