Viral and cellular MARCH ubiquitin ligases and cancer

Abstract

Covalent conjugation of proteins with ubiquitin is one the most important post-translational modifications because it controls intracellular protein trafficking typically resulting in protein degradation. Frequently ubiquitinated proteins are targeted to the proteasome for degradation in the cytosol. However, ubiquitinated membrane bound proteins can also be targeted for endocytosis and degradation in the lysosome. Ubiquitin-dependent degradation pathways have clear cancer relevance due to their integral involvement in protein quality control, regulation of immune responses, signal transduction, and cell cycle regulation. In spite of its fundamental importance, little is known regarding how proteins are specifically identified for ubiquitin-dependent degradation. In this article we review a newly discovered family of viral and cellular ubiquitin ligases called MARCH proteins. Recent studies of MARCH proteins define new paradigms showing how ubiquitin E3 ligases determine the intracellular location and fate of proteins.

Figure 1. ERAD pathway induced by viral ligase mK3

A. mK3 binds to its primary binding partner TAP that is associated with the class I specific chaperone tapasin. B. After assembly with β2m, the MHC class I heavy chain (HC) binds the tapasin/TAP complex to await a suitable peptide ligand. C. The association of mK3 with TAP/tapasin induces the recruitment of an E2 conjugase and polyubiquitination of the HC tail. β2m also dissociates from HC perhaps bringing the ER chaperone calnexin (CXN) into the multimeric complex as shown in the next panel. D. Polyubiquitination of the HC initiates partial denaturation of HC and the recruitment of the ATPases p97. Then p97 facilitates the retro-translocation of HC to the cytosol via a putative dislocation channel that potentially includes Derlin and mK3. E. HC undergoes proteasome-mediated degradation in the cytosol as chaperoned by p97, resulting in ubiquitin monomers and peptides.

Figure 2. Evolutionary relationships of murine and viral RING-CH domains

Amino acid sequences of murine MARCH and viral RING-CH domains were aligned using the T-COFFEE algorithm, and the evolutionary history was inferred using the Neighbor-Joining method. The bootstrap consensus tree inferred from 5000 replicates is taken to represent the evolutionary history of the taxa analyzed. Branches corresponding to partitions reproduced in less than 50% bootstrap replicates are collapsed. The tree is drawn to scale, with branch lengths in the same units as those of the evolutionary distances used to infer the phylogenetic tree. The evolutionary distances were computed using the Dayhoff matrix based method and are in the units of the number of amino acid substitutions per site [104]. All positions containing alignment gaps and missing data were eliminated only in pairwise sequence comparisons (Pairwise deletion option). There were a total of 58 positions in the final dataset. Phylogenetic analyses were conducted in MEGA4 [105].

Figure 3

Aligned amino acid sequences of viral K3 and murine RING-CH domains. Using the T-COFFEE algorithm, amino acid sequences of kK3, kK5 and mK3 RING-CH domains as well as the RING-CH domains from representative MARCH proteins were aligned. Blue shading reveals the conserved cysteine and histidine residues of the C4HC3 motif responsible for coordinating two zinc ions (blue shading Fig. 3).