Deep-Sea Genome Mining Reveals Cooperative ATP-Grasp Ligase-Directed Biosynthesis of Pentacyclic Myxomiditides with Potent Protease Inhibition

Microviridins are ribosomally synthesized and post-translationally modified peptides, typically featuring a conserved tricyclic structure formed by two ATP-grasp ligases. However, the diversity and evolution of these enzymes remain incompletely understood. Here, we identify a distinct ATP-grasp ligase subclade (MyxF) that specifically modifies the conserved (KxxE)_n motif, defining a new subclass of microviridins with the (KxxE)_nTxKxPSDx-(D/E)-(D/E) sequence signature. Guided by SSN analysis, we discovered a deep-sea <i>myx</i> biosynthetic gene cluster from 10,000 m sediments and heterologously expressed two pentacyclic microviridin-like peptides, Myxomiditide A and B. Using mass spectrometry and NMR, we fully elucidated their chemical structures, revealing not only the conserved tricyclic core but also two additional N-terminal lactam rings within the KxxEKxxE motif, distinguishing them from known microviridins. Combined <i>in vivo</i> coexpression and <i>in vitro</i> reconstitution uncovered a noncanonical division of labor among four ATP-grasp ligases involved in myxomiditide biosynthesis. MyxF and MyxD1 act as functional isozymes responsible for installation of the N-terminal lactam moieties, whereas MyxD2catalytically inactive on its ownrequires the synergistic presence of both MyxF and MyxD1 to enable formation of the C-terminal lactone rings. The pathway is finalized by MyxC, which catalyzes the terminal lactam macrocyclization, collectively revealing a highly cooperative enzymatic assembly mechanism governing myxomiditide maturation. Furthermore, MyxF exhibited remarkable catalytic plasticity, catalyzing multiple lactam macrocyclizations beyond its native substrate architecture. Notably, Myxomiditide A potently inhibited elastase with nanomolar IC₅₀ values. Collectively, this study expands the enzymatic landscape of ATP-grasp ligases and highlights the deep sea as a rich source of evolutionary innovation in RiPP biosynthesis.