FBXW4

FBXW4 (dactylin; SHFM3) is a member of the F-box/WD40 (FBXW) family of proteins. Its 412-residue product contains an N-terminal F-box motif (approximately residues 25-71) and a C-terminal beta-propeller built from roughly six WD40 repeats. F-box proteins function as the interchangeable substrate-recognition subunits of SCF (SKP1-CUL1-F-box)-type cullin-RING E3 ubiquitin ligase complexes: the F-box motif docks the protein onto SKP1/CUL1, while the WD40 propeller recognizes (typically phosphorylated) substrate proteins and presents them for ubiquitination by the RBX1-bound E2, committing them to proteasomal degradation. Affinity-purification studies confirm that FBXW4 assembles, in an F-box-dependent manner, with the canonical SCF core subunits SKP1, CUL1 and RBX1 and with COP9 signalosome subunits (which deneddylate and regulate cullin-RING ligases), and that FBXW4 associates with ubiquitinated cellular proteins in a manner enhanced by proteasome inhibition - establishing it as a bona fide SCF substrate receptor engaged in ubiquitin-dependent turnover. Its physiological substrates, however, remain undefined. FBXW4 is broadly expressed (including brain, kidney, lung and liver) and is recurrently mutated, deleted, or under-expressed across human cancers, suggesting a candidate tumor-suppressor role. The human gene maps to chromosome 10q24.3 within the critical region for split-hand/foot malformation type 3 (SHFM3), and the orthologous gene is disrupted in the mouse dactylaplasia (Dac) mutant, which fails to maintain the apical ectodermal ridge and truncates the autopod. SHFM3 is most often associated with tandem/discontinuous genomic duplications at 10q24 that perturb a shared cis-regulatory neighborhood and alter expression of FBXW4 and several neighboring genes (e.g. BTRC, POLL, LBX1) rather than with coding mutations in FBXW4 itself, so FBXW4's precise contribution to limb development and its physiological ubiquitination substrates remain incompletely defined.