The YukD protein family members participate in the formation of a translocon required for the secretion of WXG100 proteins (Pfam:PF06013) in monoderm bacteria, with the WXG100 protein secretion system (Wss). Like the cytoplasmic protein EsaC in Staphylococcus aureus, YukD was hypothesized to play a role of a chaperone. YukD adopts a ubiquitin-like fold [1]. Usually, ubiquitin covalently binds to protein and flags them for protein degradation, however conjugation assays have indicated that the classical YukD lacks the capacity for covalent bond formation with other proteins [1]. In contrast to the situation in firmicutes, YukD-like proteins in actinobacteria are often fused to a transporter involved in the ESAT-6/ESX/Wss secretion pathway [6,7]. Members of the YukD family are also associated in gene neighborhoods with other enzymatic members of the ubiquitin signaling and degradation pathway such as the E1, E2 and E3 trienzyme complex that catalyze ubiquitin transfer to substrates, and the JAB family metallopeptidases that are involved in its release [7]. This suggests that a subset of the YukD family in bacteria are conjugated and released from proteins as in the eukaryotic ubiquitin-mediated signaling and degradation pathway [7].

1: Crystal structure of the ubiquitin-like protein YukD from Bacillus subtilis. van den Ent F, Lowe J; FEBS Lett. 2005;579:3837-3841. PMID:15978580

2: EsxA and EsxB are secreted by an ESAT-6-like system that is required for the pathogenesis of Staphylococcus aureus infections. Burts ML, Williams WA, DeBord K, Missiakas DM; Proc Natl Acad Sci U S A. 2005;102:1169-1174. PMID:15657139

3: Outer membrane translocation: numerical protein secretion nomenclature in question in mycobacteria. Desvaux M, Hebraud M, Talon R, Henderson IR; Trends Microbiol. 2009;17:338-340. PMID:19674902

4: New insights into the distribution of WXG100 protein secretion systems. Sutcliffe IC; Antonie Van Leeuwenhoek. 2011;99:127-131. PMID:20852931

5: Secretion and subcellular localizations of bacterial proteins: a semantic awareness issue. Desvaux M, Hebraud M, Talon R, Henderson IR; Trends Microbiol. 2009;17:139-145. PMID:19299134

6: The prokaryotic antecedents of the ubiquitin-signaling system and the early evolution of ubiquitin-like beta-grasp domains. Iyer LM, Burroughs AM, Aravind L; Genome Biol. 2006;7:R60. PMID:16859499

7: Functional diversification of the RING finger and other binuclear treble clef domains in prokaryotes and the early evolution of the ubiquitin system. Burroughs AM, Iyer LM, Aravind L; Mol Biosyst. 2011;7:2261-2277. PMID:21547297