Figure 4. J-domain and client protein binding domain structures.

a| J-domains contain four alpha-helices, with the central ones forming a coiled-coil motif around a hydrophobic core¹³⁷. The invariant HPD tripeptide located in the loop between helices II and III is critical for ATPase stimulation and in vivo function¹². Residues in helix II and within the neighboring loop, including the HPD, form an Hsp70 interaction face. b| Class I Ydj1 and Class II Sis1 have similar client protein binding domains, the “DnaJ-type”. The structure of amino acids 102-350 of Ydj1²⁶, ²⁹ and amino acids 180-343 of Sis1³⁰ are shown. Both have J-domains at their N-termini, followed by a glycine/phenyalanine-rich (G/F) region. No full-length structure of a Class I or Class II J-protein has been obtained, presumably because of the flexibility of the G/F regions. Both Ydj1 and Sis1 are dimers, with the dimerization domain at their C-termini (C). Each monomer of Ydj1 and Sis1 has two adjacent domains that are similar structure, being predominantly composed of beta-sheets. These are often referred to as “C-terminal domain” (CTD1) and CTD2. Ydj1 (like DnaJ) has two zinc fingers (Zn1 and Zn2), which extend out from CTD1. In addition, Ydj1 has a CAAX motif for farnesylation, a modification important for membrane localization and binding of some client proteins^{138 139}. c|Jac1 (called HscB in E. coli), which is critical for Fe-S cluster biogenesis, has a specialized client protein binding domain that has neither sequence nor structural similarities to a DnaJ-type¹⁴⁰; amino acids 63-171 of HscB are shown. The face pointing outwards interacts with the Fe-S cluster scaffold Isu¹⁴¹.