We have identified three new
tumor necrosis factor-receptor associated factor (TRAF) domain-containing
proteins in humans using bioinformatics approaches, including: MUL, the product of the causative gene in
Mulibrey Nanism syndrome; USP7 (HAUSP), an
ubiquitin protease; and SPOP, a POZ domain-containing
protein. Unlike classical TRAF family
proteins involved in TNF family receptor (TNFR) signaling, the TRAF domains (
TDs) of MUL, USP7, and SPOP are located near the NH(2) termini or central region of these
proteins, rather than carboxyl end. MUL and USP7 are capable of binding in vitro via their
TDs to all of the previously identified TRAF family
proteins (
TRAF1,
TRAF2,
TRAF3,
TRAF4,
TRAF5, and
TRAF6), whereas the TD of SPOP interacts weakly with
TRAF1 and
TRAF6 only. The TD of MUL also interacted with itself, whereas the
TDs of USP7 and SPOP did not self-associate. Analysis of various MUL and USP7 mutants by transient transfection assays indicated that the
TDs of these
proteins are necessary and sufficient for suppressing
NF-kappaB induction by
TRAF2 and
TRAF6 as well as certain TRAF-binding TNF family receptors. In contrast, the TD of SPOP did not inhibit
NF-kappaB induction. Immunofluorescence confocal microscopy indicated that MUL localizes to cytosolic bodies, with targeting to these structures mediated by a RBCC tripartite domain within the MUL
protein. USP7 localized predominantly to the nucleus, in a TD-dependent manner. Data base searches revealed multiple
proteins containing
TDs homologous to those found in MUL, USP7, and SPOP throughout eukaryotes, including yeast, protists, plants, invertebrates, and mammals, suggesting that this branch of the TD family arose from an ancient gene. We propose the moniker TEFs (TD-encompassing factors) for this large family of
proteins.