Organelle formation from scratch

Cellular organelles called peroxisomes aid metabolism, and defective peroxisome formation can cause disease. It emerges that peroxisomes can form de novo from the fusion of vesicles derived from two distinct organelle types. See Letter p.251 Peroxisomes are home to a variety of metabolic enzymes. These membrane-bound organelles work closely with mitochondria, exchanging metabolites for completion of a number of enzymatic cascades. Peroxisomes can either appear anew or divide from pre-existing organelles, specifically the endoplasmic reticulum. Intrigued by the knowledge that in the absence of peroxisomes, several integral peroxisomal membrane proteins are imported into the mitochondria of mammalian cells, Heidi McBride and co-workers investigate the role of mitochondria in the formation of peroxisomes. They find that in mammalian cells, peroxisomes are the product of structures that arise from not just the endoplasmic reticulum, but also from mitochondria.