The N-domain of Pex22p can functionally replace the Pex3p N-domain in targeting and peroxisome formation.

Pex3p is a central component of the import machinery for peroxisomal membrane proteins (PMPs) that can reach peroxisomes via the endoplasmic reticulum (ER) and even trigger de novo peroxisome formation from the ER. Pex19p is the import receptor for type I PMPs, whereas targeting of type II PMPs, of which Pex3p so far represents the only species, does not require Pex19p. Pex3p possesses two domains with distinct function: a short N-terminal domain, which harbors the information for peroxisomal (and ER) targeting, and a C-terminal domain, which faces the cytosol and serves as a docking site for Pex19p, thereby delivering newly synthesized PMPs to the peroxisome. Here we show that the N-terminal domain of Pex3p can be functionally replaced by the N-terminal peroxisomal membrane targeting signal (mPTS) of Pex22p, a supposedly unrelated component of the import machinery for peroxisomal matrix proteins. An exchange of the mPTS of Pex22p by that of Pex3p likewise fully preserved the function of Pex22p. Neither of the two mPTS interacted with Pex19p, and in the absence of Pex19p, colocalization of Pex3p and Pex22p was observed, indicating that also Pex22p is targeted to peroxisomes by a type II mPTS. When a type I mPTS was hooked to the C-terminal domains of Pex22p and Pex3p, function was retained in the case of Pex22p and in part even for Pex3p. The C-terminal domain of Pex3p thus contains the relevant information required for de novo peroxisome formation, thereby challenging the concept of the N terminus of Pex3p being key in that process.

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