To establish a life-long infection, herpesviruses have evolved means of subverting normal host defenses. We, and others, have found that Kaposi's Sarcoma associated herpesvirus (KSHV) encodes two proteins, MIR1 and MIR2, that prevent antigen presentation by MHC-I molecules. These proteins decrease MHC-I expression on the surface of the infected cell, thus preventing recognition by cytotoxic T lymphocytes (CTLs). These MIR proteins act as E3 ubiquin ligases of the ubiquitin pathway. They promote ubiquitination of the MHC-I intracytoplasmic domain, a signal that targets MHC-I molecules for internalization from the cell surface and degradation by the lysosomes. . Lysine residues and the N-terminus were long thought to be the exclusive sites of ubiquitin attachment on substrates. Unexpectedly, we recently observed that the MIR proteins have the unique ability to ubiquitinate their substrates on cysteine residues. Whether this novel form of ubiquitination is specific to a few highly specialized E3s (and thus has evolved as a functional requirement) or if it is a more generalized process is still an open question.
In Aim#1, we propose to further study this novel form of ubiquitination and to identify MIR substrates regulated through this mechanism.
In Aim#2, we propose to characterize how the MIR proteins regulate the ubiquitination of their substrates. This will not only shed light into the biology of the viral MIR proteins but more broadly into the mechanism of polyubiquitination by cellular E3 ubiquin ligases.
Kaposi's Sarcoma-associated herpesvirus (KSHV) is associated with the development of B-cell lymphomas as well as the angioproliferative neoplasm Kaposi's sarcoma. We and others have previously shown that KSHV encodes two proteins that contribute to evasion of host immunity. These proteins (called MIR1 and MIR2) induce downregulation of key components of immune recognition by specifically promoting their ubiquitination and endocytosis. So far, ubiquitination, the covalent attachment of an ubiquitn protein to a substrate, was thought to occur exclusively on lysine residues.We have shown that the MIR proteins have the unique ability to mediate ubiquitination on cysteine residues. The objectives of this proposal are (1) to further characterize this novel form of ubiquitination and (2) to identify the cellular proteins interacting with the MIRs and determine how they contribute to MIR-mediated ubiquitination.
