This is a zinc knuckle motif found in PRDM4 (Schwann cell factor 1, SC-1), a member of the PR protein family. PRDM4 is a transcriptional regulator that has been implied in transduction of nerve growth factor signals via the p75 neurotrophin receptor and in cell growth arrest. The short motif is also present in several other PR proteins including human PRDM6 (PRISM), PRDM7, PRDM9 (meisetz), PRDM10 (tristanin), PRDM11, and PRDM15. The conservation of cysteine and histidine residues suggested that this 20 amino acid motif binds zinc, hence the name 'PR zinc knuckle' to distinguish it from the longer (30 amino acid) C2H2-like zinc fingers that are located C-terminally of the PR domain. The PR zinc knuckle fold is similar to that of Gag-knuckles (a beta-hairpin providing two zinc ligands followed by a short helix or a loop providing the other two zinc ligands) and zinc ribbons (two beta-hairpins, each providing two zinc ligands) [1].

1: The PR/SET domain in PRDM4 is preceded by a zinc knuckle. Briknarova K, Atwater DZ, Glicken JM, Maynard SJ, Ness TE; Proteins. 2011;79:2341-2345. PMID:21604305