Biochimica et Biophysica Acta, 1131(1992) 133-138 133 © 1992 Elsevier Science Publishers B.V. All rights reserved 0167-4781/92/$05.00 BBAEXP 92380 Ferritin mRNAs on rat liver membrane-bound polysomes synthesize ferritin that does not translocate across membranes Lorenza Tacchini, Emilia Rappocciolo, Marina Ferrero, Luisa Schiaffonati and Gaetano Cairo lstituto di Patologia Generale dell'Unit,ersith degli Studi e Centro di Studio sulla Patologia Cellulare del CNR, Milano (Italy) (Received 12 November 1991) (Revised manuscript received 27 January 1992) Key words: Ferritin; Polysome;Endoplasmic reticulum; Translocation; (Rat liver) Ferritin is a typical intracellular protein but small amounts are also present in serum and other biological fluids. The source and physiological significance of serum ferritin are still obscure. The presence of ferritin mRNAs on polysomes bound to endoplasmic reticulum (ER) could be relevant for the secretion of ferritin. By Northern blot analysis we found significant amounts of both L and H subunit mRNAs on rat liver membrane-bound polysomes. Immunoprecipitation of translational products of membrane-bound polysomes with anti-rat liver ferritin antibody showed that ferritin is actually synthesized on ER membranes. Analysis of RNA extracted from salt-washed rat liver microsomes demonstrated that ferritin mRNAs are translated by polysomes tightly bound to ER membranes. Following iron treatment, both the amount of H and L subunit mRNAs and ferritin synthesis increased sharply in both free and bound polysomal fractions. Translation of membrane-bound polysomes in the presence of microsomal membranes indicated that ferritin is not processed by signal sequence cleavage or glycosylation and is not translocated into ER membranes. Ferritin mRNAs found on membrane-bound polysomes are associated with ER in a specific way, however, their products do not seem to follow the classic secretory pathway and therefore the significance of the large amount of ferritin mRNAs in the bound ribosome fraction remains unclear. Introduction Ferritin is an iron storage protein composed of a shell of 24 subunits of two types, H and L. The assem- bly in varying proportions of the two subunits origi- nates families of molecules, isoferritins, with different tissue specificity, metabolism and function (see Refs. 1 and 2 for review). Most ferritin is localized intra- cellularly but small amounts are also present in serum [3] and other biological fluids [4]. The high correlation of serum ferritin level with body iron stores [3] and its gross elevation in malignant disorders [5,6], inflamma- tory states [7,8] and iron overload [9] has made the immunological determination of ferritin a routinary Abbreviations: H. ferritin heavy chain; L. ferritin light chain; ER, endoplasmic reticulum. Correspondence to: G. Cairo, Istituto di Patologia Generale e Cen- tro di Studio sulla Patologia Cellulare del CNR, Via Mangiagalli 31, 20133, Milano, Italy. clinical practice. However, the source and physiological significance of serum ferritin are still obscure: both cells of the reticuloendothelial system and liver parenchimal cells have been indicated as best candi- dates for the production of circulating ferritin. In fact, small amounts of concanavalin-A-binding ferritin were detected during incubation of phagocytic blood cells [10] and released ferritin was found in experiments with perfused rat liver [11]. In addition, evidence for ferritin release from T lymphocyte [12] and tumour cells [13,14] was provided. In these studies the molecu- lar details of the secretory pathway of ferritin were not investigated, nevertheless, several additional evidences argue in favour of a specific secretion rather than accidental leakage from cells. In fact, contrary to its intracellular counterpart, serum ferritin is iron-poor [15] and partially glycosylated [16,17]. Moreover, recep- tors for ferritin have been found in several cell types [18-22] and ferritin seems to have a role in immuno- suppression [23] and regulation of myelopoiesis [24,25]. The presence of ferritin in serum is not restricted to man; in fact, circulating ferritin was found in the rat