The J-protein family: modulating protein assembly, disassembly and translocation

Abstract

DnaJ is a molecular chaperone and the prototypical member of the J-protein family. J proteins are defined by the presence of a J domain that can regulate the activity of 70-kDa heat-shock proteins. Sequence analysis on the genome of Saccharomyces cerevisiae has revealed 22 proteins that establish four distinguishing structural features of the J domain: predicted helicity in segments I-IV, precisely placed interhelical contact residues, a lysine-rich surface on helix II and placement of the diagnostic sequence HPD between the predicted helices II and III. We suggest that this definition of the J-protein family could be used for other genome-wide studies. In addition, three J-like proteins were identified in yeast that contain regions closely resembling a J domain, but in which the HPD motif is non-conservatively replaced. We suggest that J-like proteins might function to regulate the activity of bona fide J proteins during protein translocation, assembly and disassembly.

Figure 1

Multiple sequence alignment of the J domains of 22 yeast proteins. Colour coding indicates chemically similar residues as follows: orange, basic; blue, acidic; green, polar; red, non-polar. Grey boxes indicate the extent of helical segments predicted in each yeast protein (see the PredictProtein server online at http://www.cs.bgu.ac.il/~dfischer/predictprotein/#) that correspond to helices I–IV of DnaJ and Hdj1 (Szyperski et al, 1994; Qian et al, 1996). Asterisks represent hydrophobic residues that are important for helix packing and stability. The numbers indicate the length of the J domain from each protein.

Figure 2

Structural classification of J proteins. Representation of type I, II and III J proteins from yeast aligned according to the amino-terminus of the mature protein. Mdj1 and Scj1 have targeting sequences processed in the mitochondria and endoplasmic reticulum, respectively. The grey boxes represent each polypeptide and show the scale of the J domain (J, orange), glycine-rich region (G, yellow) and zinc-finger domain (Zn-finger, green) found in some J proteins compared with the protein–protein-interaction domains that bind non-native substrate. CTD, carboxy-terminal domain; TM, transmembrane segment.

Figure 3

Subcellular location and organization of J proteins in yeast. The location of the 22 proteins is shown on the left (see Table 1 for identities) and the inset section has been magnified to detail the J proteins in the cytosol. Most are free in the cytosol and dynamically associate with subcellular structures, although some, such as Hlj1 and perhaps Jid1 and Jjj1, are tethered to a specific membrane facing the cytosol.