Cbl exposes its RING finger

Abstract

The Cbl family of RING finger ubiquitin ligases regulates signaling in many systems. Two new studies provide a structural basis for how phosphorylation of a specific tyrosine in the Cbl proteins enhances their ubiquitin ligase activity, giving insight into how ubiquitination by Cbl proteins is restricted to specific substrates.

Figure 1

The Cbl proteins. There are three mammalian Cbl proteins, encoded by separate genes: Cbl (also known as c-Cbl, CBL2 or RNF55), Cbl-b (also known as RNF56) and Cbl-c (also known as Cbl-3, Cbl-SL or RNF57). D-Cbl_L and D-Cbl_S are the long and short spliced isoforms, respectively, of the Drosophila melanogaster Cbl protein. Sli-1 is the Caenorhabditis elegans Cbl protein. All Cbl proteins contain a highly conserved N-terminal TKB domain, a linker region (L) and a C3HC4 RING finger (RF). The C termini of the Cbl proteins contain tyrosines (Y) that can become phosphorylated and mediate interactions with SH2-containing proteins. The C termini also contain proline-rich regions (P) that mediate interactions with SH3-containing proteins. The longer forms of Cbl proteins have a ubiquitin-associated (UBA) domain at the C terminus. 4H, four-helix bundle.

Figure 2

Model of Cbl function. (1) Prior to interaction with a substrate, Cbl proteins exist in an inactive closed conformation in which the TKB domain interacts with the RING finger in a tightly packed structure that blocks E2 binding. This form is in equilibrium with a partially unfolded structure. The ubiquitin-charged E2 can bind to the partially unfolded Cbl RING finger with low affinity, butthe Cbl protein is inactive. (2) Upon activation and autophosphorylation of the RTK, the Cbl protein interacts with a phosphorylated tyrosine on the RTK through the TKB. This interaction may either stabilize or select for the partially relaxed structure. (3) The Cbl protein is phosphorylated on the linker tyrosine, causing a rotation of the linker region and thus exposing the RING finger for higher-affinity binding of the ubiquitin-charged E2 and bringing the E2 into close proximity to the substrate. (4) The Cbl protein is now active and enhances transfer of ubiquitin from the bound E2 to the substrate. (5) The ubiquitination of the RTK leads to trafficking of the RTK through the endocytic system to the lysosome for degradation. RF, RING finger; P, proline-rich regions; UBA, ubiquitin-associated domain; Y, linker tyrosine residue; Ub, ubiquitin; PO₄, phosphate groups. The linker is depicted as a helix, and the linker tyrosine is indicated.