The protein of M_r 21 000 constituting the prosome‐like particle of duck erythroblasts is homologous to apoferritin

Abstract

In duck erythroblasts, two major populations of untranslated messenger (m) RNP can be separated by sucrose gradient centrifugation in low ionic strength. One of these contains globin mRNA associated to protein factors, among them the prosomes. The other, sedimenting in the 35S zone, contains non‐globin mRNA. From this ‘35S’ mRNP, a new RNP particle called the prosome‐like particle was isolated and characterized [Akhayat, O., Infante, A. A., Infante, D., Martins de Sa, C., Grossi de Sa, M.‐F. & Scherrer, K. (1987) Eur. J. Biochem. 170, 23–33]. The PLP is a multimer of a protein of Mr 21000, and contains small RNA species. The particle is tightly associated with repressed mRNA and inhibits in vitro protein synthesis. We show here that the protein of Mr 21000, constituting the prosome‐like particle, is apoferritin. Different approaches confirm the RNP character of this particle and provide evidence that some of its RNA species are tRNA. The hypothesis is discussed as to whether (apo–)ferritin might serve other functions in addition to iron storage.

Purification of a RNA-binding Protein from Rat Liver

“…For example, addition of ferritin-containing prosome-like particles to an in vitro translation system inhibited the translation of globin mRNA, indicating that ferritin functioned as a translational repressor (24,29). This had also been suggested in earlier reports, which claimed that ferritin bound to the 5Ј-UTR of its own mRNA and stored the mRNA in a translational incompetent form (42).…”

“…A prosome-like particle, which was believed to play a role in protein degradation, was isolated and turned out to be composed of identical apoferritin subunits. This particle was associated with small RNA molecules, probably various tRNA species (24). These RNAs were capable of hybridizing with globin mRNA (24,29).…”

“…This particle was associated with small RNA molecules, probably various tRNA species (24). These RNAs were capable of hybridizing with globin mRNA (24,29). From Artemia cysts the protein artemin was purified, which shared homology with vertebrate ferritins and probably functions as iron storage protein (30).…”

“…It is suggested that ferritin may play a regulatory role in PCK mRNA degradation as well as in the general cytosolic mRNA turnover. The identification of ferritin as RNA-binding protein is discussed in view of an old hypothesis and recent publications, which proposed ferritin to play a role in the general metabolism of cytosolic RNA (23,24).…”

“…Due to its similar amino acid composition and immunological cross-reactivity the RNA-binding protein from Artemia cysts had been suggested to be related to the elongation factor eEF-Ts, which, however, remains to be experimentally established (31). These data suggest that ferritin, besides its function as iron storage protein, may not have a unique but rather a general function in the regulation of RNA metabolism at various levels (23,24).…”