The crystal structures of Thermus thermophilus CMP kinase complexed with a phosphoryl group acceptor and donor

Characterizing a novel CMK-EngA fusion protein from Bifidobacterium: Implications for inter-domain regulation

EngA is an essential and unique bacterial GTPase involved in ribosome biogenesis. The essentiality and species-specific variations among EngA homologues make the protein a potential target for future drug development. In this aspect, it is important to understand the variations of EngA among probiotic organisms and non-probiotic bacteria to understand species specificity. The search for variations among EngA homologues revealed a unique variant, exclusively found in Bifidobacterium and a few Actinobacteria species. Bifidobacterium possesses a multifunctional fusion protein, wherein EngA is fused with an N-terminal CMK (Cytidylate Monophosphate Kinase) domain. The resulting protein is therefore a large (70kDa size) with 3 consecutive P-loops and a 50 amino acid long linker connecting the EngA and CMK domains. EngA is known to regulate ribosome biogenesis via nucleotide-dependent conformational changes. The additional domain may introduce further intricate regulation in ribosome biogenesis or participate in newer biological processes. This study is the first attempt to characterise this novel class of bacterial EngA found in the Genus of Bifidobacteria.

The mechanism of Raf activation through dimerization

Raf, a threonine/serine kinase in the Raf/MEK/ERK pathway, regulates cell proliferation. Raf's full activation requires dimerization. Aberrant activation through dimerization is an important therapeutic target. Despite its clinical importance, fundamental questions, such as how the side-to-side dimerization promotes the OFF-to-ON transition of Raf's kinase domain and how the fully activated ON-state kinase domain is stabilized in the dimer for Raf signaling, remain unanswered. Herein, we decipher an atomic-level mechanism of Raf activation through dimerization, clarifying this enigma. The mechanism reveals that the replacement of intramolecular π–π stacking by intermolecular π–π stacking at the dimer interface releases the structural constraint of the αC-helix, promoting the OFF-to-ON transition. During the transition, the inhibitory hydrophobic interactions were disrupted, making the phosphorylation sites in A-loop approach the HRD motif for cis-autophosphorylation. Once fully activated, the ON-state kinase domain can be stabilized by a newly identified functional N-terminal basic (NtB) motif in the dimer for Raf signaling. This work provides atomic level insight into critical steps in Raf activation and outlines a new venue for drug discovery against Raf dimerization.

We decipher an atomic-level mechanism of Raf activation through dimerization, revealing that the disruption of intramolecular π–π stacking at the dimer interface promotes the OFF-to-ON transition.