Even though new peroxisomes can form from the ER, faithful segregation of these, like other, organelles during cell division is necessary. For multi-copy organelles, the traditional view has been that stochastic principles suffice. However, it is now becoming clear that strict rules govern the inheritance of non-nuclear organelles, which require many accessory factors, such as cytoskeletal proteins, motor proteins and specific linker proteins. Certainly, in the case of autonomously multiplying organelles, a suite of proteins are required for organellar fission to maintain their numbers (Okamoto and Shaw, 2005).
Peroxisomes turn out to be no exception to these rules. S. cerevisiae is an attractive model for studies of peroxisome partitioning because its polar bud growth necessitates extra care to make sure that each bud receives a full complement of organelles. Real-time imaging of yeast with peroxisomes labeled with CFP-PTS1 over several division cycles indicated that peroxisomes are equally distributed between mother and daughter cells (Hoepfner et al., 2001). During partitioning, part of the peroxisome population remains relatively statically associated with the cortex of the mother cell, whereas the rest moves in a more dynamic way towards and into the bud. Deleting the gene encoding the dynamin-like protein Vps1p (see below) results in a single, giant peroxisome per cell. Surprisingly, the single peroxisome is faithfully distributed over the next generations of cells, mimicking the precision of nuclear division. In common with the transport of mitochondria, vacuoles and secretory vesicles along the actin cytoskeleton in yeast, the myosin motor Myo2p carries peroxisomes along actin cables into the bud.
Recently, a new participant in this process was described (Fagarasanu et al., 2005). Inp1p is a peroxisome-associated protein identified in a genome-wide screen assigning subcellular locations to yeast open reading frames (Huh et al., 2003). Deletion of INP1 or its overexpression has dramatic effects on peroxisome partitioning. In its absence, almost all the peroxisomes move into the bud. However, upon its overexpression, they all remain in the mother cell close to the cortex. Overexpressed INP1 is associated with both peroxisomes and the cortex. Fagarasanu et al. suggest that it tethers some of the peroxisomes to the mother cell cortex to immobilize them and prevent them from entering the bud. To leave some peroxisomes free to move to the bud, Inp1p levels must be critically controlled. Indeed, Fagarasanu et al. find that they fluctuate during the cell cycle.
It is clear that additional proteins that remain to be identified are involved in peroxisome partitioning. Cortical protein(s) must bind to Inp1p, for example, and a (membrane) protein probably anchors Myo2p to peroxisomes. The involvement of the actin cytoskeleton may be particular to S. cerevisiae. In mammalian cells, peroxisomes move along microtubules, propelled by dynein or kinesin motor proteins. Whether these are responsible for peroxisome partitioning during mammalian cell division is not yet clear.