The Structural Versatility of the BTB Domains of KCTD Proteins and Their Recognition of the GABAB Receptor

Several recent investigations have demonstrated that members of the KCTD (Potassium Channel Tetramerization Domain) protein family are involved in fundamental processes. However, the paucity of structural data available on these proteins has frequently prevented the definition of their biochemical role(s). Fortunately, this scenario is rapidly changing as, in very recent years, several crystallographic structures have been reported. Although these investigations have provided very important insights into the function of KCTDs, they have also raised some puzzling issues. One is related to the observation that the BTB (broad-complex, tramtrack, and bric-à-brac) domain of these proteins presents a remarkable structural versatility, being able to adopt a variety of oligomeric states. To gain insights into this intriguing aspect, we performed extensive molecular dynamics simulations on several BTB domains of KCTD proteins in different oligomeric states (monomers, dimers, tetramers, and open/close pentamers). These studies indicate that KCTD-BTB domains are stable in the simulation timescales, even in their monomeric forms. Moreover, simulations also show that the dynamic behavior of open pentameric states is strictly related to their functional roles and that different KCTDs may form stable hetero-oligomers. Molecular dynamics (MD) simulations also provided a dynamic view of the complex formed by KCTD16 and the GABAB2 receptor, whose structure has been recently reported. Finally, simulations carried out on the isolated fragment of the GABAB2 receptor that binds KCTD16 indicate that it is able to assume the local conformation required for the binding to KCTD.

Studying interactions between the Mod(mdg4)-67.2 protein and other isoforms of the Mod(mdg4) in the embryonic cells of Drosophila melanogaster

It was found that in the embryonic cells of D. melanogaster, the isoforms of the Mod(mdg4) protein is able to interact with each other through the BTB domains. However, this non-specific interaction is destroyed when protein complexes recruiting to chromatin sites.

Btbd6-dependent Plzf recruitment to Cul3 E3 ligase complexes through BTB domain heterodimerization

The Cul3 adaptor Btbd6 plays crucial roles in neural development by driving the ubiquitin-dependent degradation of promyelocytic zinc finger transcription factor (Plzf). Btbd6 has conserved motifs, BTB-BACK-PHR, and by analogy with other BTB-BACK adaptors, might be expected to bind to Cul3 through the BTB-BACK domain, and to substrate through the PHR domain. However, we now present a mode of adaptor-substrate interaction through heterodimerisation between the normally homodimeric BTB domains of Btbd6 and Plzf. This heterodimerization appears to occur through monomer exchange that is detected only at or near physiological concentrations. The Btbd6-Plzf heterodimer thus formed assembles into a ternary complex with Cul3. In addition we show that the BTB and PHR domains of Btbd6 promote localisation in the nucleus and that the BACK domain contains a nuclear export signal. Our findings support a model whereby Btbd6 moves into and out of the nucleus, iteratively ‘sweeping’ Plzf into the cytoplasm and enabling complex formation with Cul3 that presents Plzf for ubiquitination.HighlightsA general mechanism for recruitment of BTB domain-containing substrates by BTBdomain adaptors for the Cul3 E3 ligase complexNuclear export of the Plzf/Btbd6 complex mediated by a NES within the Btbd6 BACK domainCul3-dependent Plzf ubiquitylation through heterodimerisation of BTB domains on adaptor and substrate by monomer exchang