Mature peroxisomes fractionate as a homogeneous species, but various subsets of peroxisomal proteins can be found in other biochemical fractions. Now Titorenko et al. (page 29) show that these subsets represent precursors in peroxisome formation. They characterize six distinct peroxisome species in the yeast Yarrowia lipolytica, from the most immature (P1) to the most mature (P6), and find that an early step in peroxisome formation involves vesicular fusion.
By pulse–chase, proteins appear first in the P1 and P2 fractions, before progressing sequentially through the other stages. P1 and P2 have distinct protein compositions, and they fuse in vitro to form a set of vesicles that appear identical, by several measures, to the P3 species formed in vivo.
As the peroxisomes mature, sequential import provides them with an increasing fraction of the proteins present in mature peroxisomes. The various species of peroxisomes are...
