Friday, May 14, 2010

Outsourcing Biogenesis

Role of ER in peroxisome biogenesis in native conditions in wild type cells is quiet debated. Several reports on mitochondrial proteins residing also on peroxisomes are emerging thesedays. Most recent entry in this list of dually localized proteins is MAVS (Mitochondrial Antiviral Signaling protein) which plays critical role in antiviral response (Dixit et al, Cell, 2010). Pex19 dependant targeting of mitochondrial tail anchored proteins to peroxisomal membrane is confirmed in case of Fis1 (Dellile et al, JBC, 2008), Vps1 (Vizeacoumar, JBC, 2006) and partial dependancy of  Dlp1 on Pex11 for peroxisomal targeting is also reported (Li et al, JBC, 2003). Vescicular carrier mediated transport of specific cargo from mitochondria to peroxisome is also reported (Neuspiel et al, Curr. Biol, 2008).

This indeed stresses the existence of wider range of inter-organellar communication between mitochondria and peroxisomes. Especially this cooperation can be at peak level in specialized cells like photocytes, the light emitting cells from fireflies. Bioluminescence in fireflies involves ATP dependant activation of Luciferin by Luciferase and its subsequent release of energy in form of light pulses. Both luciferase and luciferin are localized in peroxisomes whereas mitochondria provide ATP and regulate the oxygen supply in tracheal system surrounding photocytes. (More Info.) Complexity is more in case of plant cells where there is cross talk between chloroplast, mitochondria and peroxisomes.

Similar to peroxisomes, the biogenesis of autophagosomes is in controversy whether they are formed de novo or from pre-existing membranes. In case of yeast, autophagosomes are generated from PreAutophagosomal Structures (PAS) formed of ATG proteins near vacuaole. Whereas there was much ambiguity in knowledge about this process in mammalian system.  
                   Recent paper in Cell by Hailey et al titled “Mitochondria Supply Membranes for Autophagosome Biogenesis during Starvation” trace the autophagosomal membrane origin to mithochondria in mammalian cells (at least during starvation!). Though autophagosomal membrane can be formed from ER or plasma membrane depending upon the kind of stress. In this paper, authors offer a conclusive proof that autophagosome formation in starvation induced autophagy relies on mitochondria for the lipid components. Intriguingly only lipid components from mitochondrial outer membrane and not the mitochondrial proteins are utilized for autophagosome biogenesis. The authors present elegant experimental proofs that mitochondria-autophagosome interaction is an entirely independent and native phenomenon other than mitophagy itself.

The ability of mitochondria to provide membrane template for biogenesis of vescicles and autophagosomes reminds the peroxisome biologists that in the debate of ER-Peroxisomes, we should not miss mitochondrial role in between.

-Vishal Kalel. -

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