B-box zinc finger explained

Symbol:	zf-B_box
zf-B_box
Pfam:	PF00643
Interpro:	IPR000315
Prosite:	PDOC50015
Scop:	1fre
Cdd:	cd00021

In molecular biology the B-box-type zinc finger domain is a short protein domain of around 40 amino acid residues in length. B-box zinc fingers can be divided into two groups, where types 1 and 2 B-box domains differ in their consensus sequence and in the spacing of the 7-8 zinc-binding residues. Several proteins contain both types 1 and 2 B-boxes, suggesting some level of cooperativity between these two domains.

Occurrence

B-box domains are found in over 1500 proteins from a variety of organisms. They are found in TRIM (tripartite motif) proteins that consist of an N-terminal RING finger (originally called an A-box), followed by 1-2 B-box domains and a coiled-coil domain (also called RBCC for Ring, B-box, Coiled-Coil). TRIM proteins contain a type 2 B-box domain, and may also contain a type 1 B-box. In proteins that do not contain RING or coiled-coil domains, the B-box domain is primarily type 2. Many type 2 B-box proteins are involved in ubiquitinylation. Proteins containing a B-box zinc finger domain include transcription factors, ribonucleoproteins and proto-oncoproteins; for example, MID1, MID2, TRIM9, TNL, TRIM36, TRIM63, TRIFIC, NCL1 and CONSTANS-like proteins.^[1]

The microtubule-associated E3 ligase MID1 (EC) contains a type 1 B-box zinc finger domain. MID1 specifically binds Alpha-4, which in turn recruits the catalytic subunit of phosphatase 2A (PP2Ac). This complex is required for targeting of PP2Ac for proteasome-mediated degradation. The MID1 B-box coordinates two zinc ions and adopts a beta/beta/alpha cross-brace structure similar to that of ZZ, PHD, RING and FYVE zinc fingers.^{[2] [3]}

Homologs

Prokaryotic homologs of the domain are present in several bacterial lineages and methanogenic archaea, and often show fusions to peptidase domains such as the rhomboid-like serine peptidase, and Zn-dependent metallopeptidase. Other versions typically contain transmembrane helices and might also show fusions to domains such as DNAJ, FHA, SH3, WD40 and tetratricopeptide repeats. Together these associations suggest a role for the prokaryotic homologs of the B-box zinc finger domain in proteolytic processing, folding or stability of membrane-associated proteins. The domain architectural syntax is remarkably similar to that seen in prokaryotic homologs of the AN1 zinc finger and LIM domains.^[4]

External links

• CO-like family, DBB family at PlantTFDB: Plant Transcription Factor Database

See also

• Zinc finger

Notes and References

1. Short KM, Cox TC . Subclassification of the RBCC/TRIM superfamily reveals a novel motif necessary for microtubule binding . J. Biol. Chem. . 281 . 13 . 8970–80 . March 2006 . 16434393 . 10.1074/jbc.M512755200 . free .
2. Massiah MA, Matts JA, Short KM, Simmons BN, Singireddy S, Yi Z, Cox TC . Solution structure of the MID1 B-box2 CHC(D/C)C(2)H(2) zinc-binding domain: insights into an evolutionarily conserved RING fold . J. Mol. Biol. . 369 . 1 . 1–10 . May 2007 . 17428496 . 10.1016/j.jmb.2007.03.017 .
3. Massiah MA, Simmons BN, Short KM, Cox TC . Solution structure of the RBCC/TRIM B-box1 domain of human MID1: B-box with a RING . J. Mol. Biol. . 358 . 2 . 532–45 . April 2006 . 16529770 . 10.1016/j.jmb.2006.02.009 .
4. Burroughs AM, Iyer LM, Aravind L . Functional diversification of the RING finger and other binuclear treble clef domains in prokaryotes and the early evolution of the ubiquitin system . Mol. Biosyst. . 7 . 1 . 2261–77 . July 2011. 21547297. 10.1039/C1MB05061C . 5938088 .