FERRITIN IN MITOCHONDRIA OF PLANT CELLS

Iron is an essential element for all living organisms. Coordinated to metallo-protein active sites, it is involved, among other, in several redox reactions linked to both oxygen production and consumption [1]. However, iron can be very toxic to cell, because of its high reactivity with hydrogen peroxide (Fenton reaction) which leads to damage of membrane lipids and other cellular components [2]. In plants, chloroplasts and mitochondria are two of the major sites of hydrogen peroxide generation [3]. To prevent this damage, two strategies can be employed: scavenging of hydrogen peroxide or sequestration of iron. Chloroplasts possess both scavenging systems (i.e. ascorbate peroxidase) [3] and proteins able to buffer iron (ferritins) [1]. Conversely, plant mitocondria have only systems to scavenge H202 or prevent its generation [4], but they do not seem to possess iron-storage proteins. Ferritins, a class of ubiquitous proteins present in fungi, animal and plant cells, can store in their core up to 4,500 atoms of iron in a safe form, thus providing a fine regulation of iron homeostasis in the cells [1]. They have been found not only in chloroplasts but, more in general, in plastids [1]. ln this work we demonstrate that ferritins are also present in plant mitochondria. By using pea stem mitochondria, isolated as previously described [5], we showed that a polyclonal antibody, raised against pea seed ferritin [6], cross-reacted with crude mitochondria (CMt) or purified mitochondria (PMt). In addition, reactivity was also found with submitochondrial particles and with matrix proteins. The bands identified corresponded to proteins with molecular masses of 29 kDa, a value which is identical to that of ferritin. PMt were devoid of contaminations from other types of membranes and, in particular, from etioplasts, as evaluated by marker enzyme determinations and electron microscope observations. A ferritin cDNA, AtFer1, has been identified from Arabidopsis thaliana seedlings [7]. The corresponding protein possesses a transit peptide responsible for plastid targeting. On the basis of this information and by means of a specific net program (www.inra.fr/servlets/WebPredotar), it was shown that a “putative ferritin” of A. thaliana, encoded by a gene which is distinct from the chloroplastic one (AtFer1), could be present in mitochondria with a score of 0.974. Therefore, to confirm the above result on pea mitochondria and the latter observation, CMt and PMt from A. thaliana cell cultures were cross-reacted with pea ferritin antibody. Again, the reaction involved a protein of about 29 kDa, thus confirming the presence of ferritins also in this type of plant material. These results show clear evidences that also plant mitochondria possess ferritins and, therefore, unveil new mechanisms of prevention of oxidative stress in plant cells. References: [1] Briat J-F and Lobréaux S. (1997), Trends in Plant Sci. 2, 187-192. [2] Halliwell B. and Gutteridge J.M.C. (1999), Free radicals in biology and medicine, 3rd edition. [3] Alscher R.G. et al. (1997), Physiol. Plant. 100, 224-233. [4] Casolo V. et al. (2000), FEBS Lett. 474, 53-57. [5] Petrussa E. et al. (2001), J. Bioenerg. Biomembr 33, 107-112. [6] Van Wuytswinkel O. et al. (2001), Plant J. 17, 93-97. [7] Gaymard F. et al. (1996), Biochem J. 318, 67-73.