Artificial import substrates reveal an omnivorous peroxisomal importomer

The peroxisome matrix protein importomer has the remarkable ability to transport oligomeric protein substrates across the bilayer. However, the selectivity and relation between import and overall peroxisome homeostasis remain unclear. Here, we microinject artificial import substrates and employ quantitative microscopy to probe limits and capabilities of the importomer. DNA and polysaccharides are "piggyback" imported when noncovalently bound by a peroxisome targeting signal (PTS)-bearing protein. A dimerization domain that can be tuned to systematically vary the binding dissociation constant (K_d ) shows that a K_d in the millimolar range is sufficient to promote piggyback import. Microinjection of import substrate at high levels results in peroxisome growth and a proportional accumulation of peroxisome membrane proteins (PMPs). However, corresponding PMP mRNAs do not accumulate, suggesting that this response is posttranscriptionally regulated. Together, our data show that the importomer can tolerate diverse macromolecular species. Coupling between matrix import and membrane biogenesis suggests that matrix protein expression levels can be sufficient to regulate peroxisome size.