Plasmodium falciparum cGMP-dependent protein kinase (PF14_0346)

The two protein kinases central in cyclic nucleotide signaling are PKA and PKG. Structural biology has been a key tool in mechanistic understanding of how PKA is activated by cAMP, particularly in elucidating the dynamic interactions between the regulatory and catalytic subunits in a tetrameric holoenzyme. PKG features a different combinatorial assembly of cyclic nucleotide binding domains (CNBs) and a kinase domain distinct from PKA, fusing them in a single protein. How PKG behaves differently from PKA as a result of this fusion is a longstanding puzzle in the study of protein kinases and second messenger signaling. We present herein the auto-inhibited structure of Plasmodium falciparum PKG, a regulator of multiple life cycle stages of the parasite responsible for malaria. This first ever structure of full-length PKG from any organism reveals four CNBs and the KD arranged in a pentagon. Some of the contiguous domains are conjoined at a single helix so as to mediate inter-domain communication and cooperativity - a feature unseen in PKA or other protein kinases and essential to the parasite's real-time response to cGMP-signaling. The KD itself differs from the catalytic subunit of PKA in two notable features: a displaced C-terminal tail and an un-phosphorylated but actively conformed activation loop - hitherto unseen in AGC kinase structures.