RING-CH finger, H2 subclass (C4HC3-type), found in Fanconi anemia group L protein (FANCL) and similar proteins
FANCL, also known as fanconi anemia-associated polypeptide of 43 kDa (FAAP43) or PHF9, is a monomeric RING E3 ubiquitin-protein ligase that monoubiquitinates FANCD2 and FANCI. The monoubiquitinated FANCD2-FANCI heterodimer complex in turn recruits key proteins involved in homologous recombination and DNA repair. FANCL is also one of seven components in Fanconi anemia (FA) nuclear core complex, which provides the essential E3 ligase function for spatially defined FANCD2 ubiquitination and FA pathway activation. In the FA core complex, FANCL associates with FANCB and FAAP100 to constitute a catalytic subcomplex that functions as the monoubiquitination module. FANCL specifically interacts with the E2 ubiquitin-conjugating (UBC) enzyme Ube2T to make an E3-E2 pair, which is the catalytic center of the Fanconi Anemia (FA) pathway required for DNA interstrand crosslink repair. Moreover, FANCL has a noncanonical function to regulate the Wnt/beta-catenin signaling, a pathway involved in hematopoietic stem cell self-renewal. It functionally enhances beta-catenin activity by ubiquitinating beta-catenin, with atypical ubiquitin chains (K11 linked). FANCL contains an N-terminal E2-like fold (ELF) domain, a novel double-RWD (DRWD) domain with a clear hydrophobic core, and a C-terminal C4HC3-type RING-CH finger. The DRWD domain is required for substrate binding. The RING-CH finger, also known as vRING or RINGv, is predicted to facilitate E2 binding. It has an unusual arrangement of zinc-coordinating residues. Its cysteines and histidines are arranged in the sequence as C4HC3-type, rather than the C3H2C3-type in canonical RING-H2 finger.
Comment:A RING finger typically binds two zinc atoms, with its Cys and/or His side chains in a unique "cross-brace" arrangement.
Comment:The RING fingers found in FANCL and its homologs have an unusual arrangement of zinc-coordinating residues: The conserved helix complete with tryptophan at the C-terminal end is present but the cysteines and histidines are arranged in the sequence as C4HC3-type, rather than the typical C3H2C3-type RING-H2 finger, and the spacing between the third and fourth cysteines is an extended 4 residues rather than the usual 2 residues.
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