Involvement of the rfp tripartite motif in protein-protein interactions and subcellular distribution

The ret finger protein (rfp) is a member of the B box zinc finger gene family which possesses a tripartite motif consisting of a RING finger, B box finger, and a coiled-coil domain. Rfp is expressed at specific stages of spermatogenesis and in various adult mouse and human tissues. It becomes oncogenic when the tripartite domain is recombined with the tyrosine kinase domain of the ret protooncogene. Many of the B box family proteins function as homodimers, although the role of the individual components of the tripartite motif in this process remains unclear. We demonstrate that rfp homomultimerization occurs through the coiled-coil domains; however, while the B box is not an interacting interface itself, its structural integrity is necessary for this interaction to occur. This is the first evidence that the B box zinc finger domain is involved in regulating protein-protein interactions. Interestingly, we find that mutations of the RING finger and B box affect the subcellular compartmentalization of rfp in various cell lines. These results demonstrate that the interactions of rfp with itself and its association with specific subcellular compartments is dependent upon the function of all of the components of the tripartite motif. It is likely that these domains play a crucial role in the function of the rfp protein in normal cell differentiation and in its transformation potential in the recombined state.