Probable substrate-specific adapter of CUL3-containing E3 ubiquitin-protein ligases which mediate the ubiquitination and subsequent proteasomal degradation of host target proteins. Interacts (via BTB domain) with host CUL3. The BTB domain is responsible for the interaction with CUL3 while the Kelch repeat domains supposely serve to recruit the cellular substrates. Belongs to the poxviruses A55 protein family.