SUMMARY PARAGRAPH for PEX21
The biogenesis of peroxisomes requires a group of protein factors referred to as peroxins which are encoded by the PEX genes. Peroxisomal proteins are synthesized on free polyribosomes and imported posttranslationally. The transport of peroxisomal matrix proteins from the cytoplasm to the peroxisome is mediated by two peroxisome-targeting signal sequences (PTS1 and PTS2). Peroxisomal membrane proteins (PMPs) are imported independently of the matrix proteins by a distinct mechanism mediated by the membrane PTS signal (mPTS) (5, 6, 7, 8 and references therein).
Pex7p, Pex18p, and Pex21p are involved in peroxisome biogenesis and the import of peroxisomal matrix proteins that contain the peroxisomal targeting sequence PTS2 (7, 9). The PTS2 is located within the first 20 amino acids and has the consensus sequence H/R-L-X5-H-L (10, 11). Pex7p binds PTS2-containing proteins in the cytoplasm and delivers them to the peroxisomal membrane where Pex18p and Pex21p contribute to their interaction with the peroxisomal import machinery (1, 2, 12).
Two subcomplexes of the peroxisomal import machinery have been defined: the docking subcomplex comprises Pex14p, Pex17p, and Pex13p, while the translocation subcomplex contains Pex2p, Pex10p, and Pex12p. The proteins of the translocation complex expose their RING finger domains to the outer face of the peroxisomal membrane, and act downstream of Pex14p, Pex17p, and Pex13p during the peroxisomal protein import process (13). Association of both subcomplexes into a larger import complex requires Pex8p, an intraperoxisomal protein. Pex8p organizes the formation of the larger import complex from the trans side of the peroxisomal membrane and thus might enable functional communication between both sides of the membrane (13).
Pex18p and Pex21p are partially redundant; import of proteins containing PTS2 is compromised only in the double mutant (1). Both Pex18p and Pex21p are constitutively degraded under normal conditions (14). Pex18p and Pex21p are not widely conserved but Pex20p in Y. lipolytica and N. crassa and Pex5L in mammals have been identified as functional homologs (15, 16).
In addition to its role in the import of peroxisomal matrix proteins, Pex21p has also been proposed to be involved in increasing the affinity of seryl-tRNA synthetase with its tRNA(Ser) substrate (17).
Last updated: 2007-06-28