20 - The Role of Shuttling Targeting Signal Receptors and Heat‐Shock Proteins in Peroxisomal Matrix Protein Import

Abstract

A number of proteins, particularly, the components of a peroxisome membrane associated complex, regulate the entry of cargoes and cargo receptors into and out of peroxisomes, and some of the proteins act catalytically to facilitate the import of proteins into the peroxisome matrix. This chapter discusses the role of peroxisome-targeting signal (PTS) receptors and the Hsp70 class of heat shock proteins (Hsp) in peroxisomal matrix protein import. Other enzymes required for peroxisomal matrix protein import include those, such as Pex4p, involved in protein ubiquitylation and the AAA ATPases, Pex1, and Pex6 that associate with each other and are needed for receptor recycling from peroxisomes to the cytosol. The receptor shuttling during peroxisomal matrix protein import involves the PTS receptors that function in multiple rounds of cargo import into peroxisomes. This mechanism allows each molecule of receptor to participate in several rounds of cargo import into peroxisomes. The chapter also presents PTS receptor-mediated steps involved in the matrix protein import cycle, and the energetics of receptor recycling and cargo import. Proteins of this family are associated with proteins being imported into the peroxisome matrix and function as ATPases such as the DnaK protein of Escherichia coli . Many members of the Hsp70 family also act in concert with nucleotide exchange factors that replace ADP with ATP, and in plants there are several reports of the existence of Hsp70 family proteins on or in peroxisomes. The peroxisomal translocon is unusual because folded, oligomeric and cofactor-bound proteins can be translocated across the peroxisomal membrane; however, the exact role of Hsp70 remained an enigma.