Production of a red blood cell's hemoglobin depends on mitochondrial heme synthesis. However, mature red blood cells are devoid of mitochondria and rely on glycolysis for ATP production. The molecular basis for the selective elimination of mitochondria from mature red blood cells remains controversial. Recent evidence suggests that clearance of both mitochondria and ribosomes, which occurs in reticulocytes following nuclear extrusion, depends on autophagy. Here, we demonstrate that Ulk1, a serine threonine kinase with homology to yeast atg1p, is a critical regulator of mitochondrial and ribosomal clearance during the final stages of erythroid maturation. However, in contrast to the core autophagy genes such as atg5 and atg7, expression of ulk1 is not essential for induction of macroautophagy in response to nutrient deprivation or for survival of newborn mice. Together, these data suggest that the ATG1 homologue, Ulk1, is a component of the selective autophagy machinery that leads to the elimination of organelles in erythroid cells rather that an essential mechanistic component of autophagy. IntroductionErythroid differentiation involves progression through morphologically distinct nucleated precursor stages, from proerythroblast to orthochromatic erythroblasts, prior to enucleation and maturation of the nascent reticulocyte. This process results in a successive reduction in cell volume, massive increase in hemoglobin production and conversion to a purely glycolytic pathway for energy production. Unlike most cells, mature red blood cells have no nucleus or organelles. Following enucleation of orthochromatic erythroblasts, nascent reticulocytes mature over the course of 48 to 72 hours and are cleared of all intracellular organelles, including mitochondria and ribosomes. Approximately 30% of red cell hemoglobin is produced in reticulocytes, and since heme is synthesized in the mitochondria these organelles are among the last to be eliminated (reviewed in Geminard et al 1 ). The programmed clearance of mitochondria that occurs in reticulocytes makes it an ideal system for studying the molecular pathways involved in mitochondrial degradation. Understanding the process by which cells degrade mitochondria is important not only for red cell maturation, but also for other cell types as accumulation of damaged or dysfunctional mitochondria has been implicated in aging and various pathologic processes such as diabetes, cancer, neurodegeneration, and ␣1 antitrypsin disease (reviewed in Pieczenik and Neustadt 2 ).The molecular pathways involved in mitochondrial degradation in reticulocytes are unknown, although several important insights have been made into this process. 15-Lipoxygenase is a lipid-peroxidizing enzyme whose expression peaks in reticulocytes shortly before organelle degradation. 3 It integrates into organelle membranes, disrupts mitochondrial membrane integrity allowing release of proteins from the organelle lumen and proteasome-dependent degradation of lumenal and integral membrane proteins. 4,5 Chemical inh...

SUMMARY Autophagy, the primary recycling pathway of cells, plays a critical role in mitochondrial quality control under normal growth conditions and in the response to cellular stress. The Hsp90-Cdc37 chaperone complex coordinately regulates the activity of select kinases to orchestrate many facets of the stress response. Although both maintain mitochondrial integrity, the relationship between Hsp90-Cdc37 and autophagy has not been well characterized. Ulk1, one of the mammalian homologues of yeast Atg1, is a serine-threonine kinase required for mitophagy. Here we show that the interaction between Ulk1 and Hsp90-Cdc37 stabilizes and activates Ulk1, which in turn is required for the phosphorylation and release of Atg13 from Ulk1, and for the recruitment of Atg13 to damaged mitochondria. Hsp90-Cdc37, Ulk1 and Atg13 phosphorylation are all required for efficient mitochondrial clearance. These findings establish a direct pathway that integrates Ulk1- and Atg13- directed mitophagy with the stress response coordinated by Hsp90 and Cdc37.