Bacterial Geosmin Biosynthesis is Compartmentalized Inside a Two-Component Protein Shell
Terpenoids are ubiquitous, structurally diverse natural products found across the three domains of life. One of the most commonly observed and well-studied terpenoids is the volatile, petrichoric compound geosmin, responsible for the earthy smell of soil to which humans are receptive down to a few parts per trillion. Bioinformatic analysis of bacterial geosmin synthase (GeoA)-encoding gene clusters reveals that many are colocalized with genes encoding Family 2B encapsulin shell proteins. Here, we focus on the encapsulin-encoding geosmin gene cluster of the model myxobacterium Myxococcus xanthus and show that the two shell proteins form a two-component Family 2B encapsulin with GeoA as the internalized cargo. Structural analysis highlights that the shell contains characteristic, external cyclic adenosine monophosphate (cAMP) binding-fold domains (CBDs), and that mixed shells can adopt a previously unobserved closed two-fold pore conformation, potentially important in cargo function modulation and regulation. We show that GeoA cargo loading is mediated by repeating, short cargo loading peptides (CLPs), and that GeoA encapsulation provides benefits for enzyme activity and stability. This work expands the short list of Family 2B encapsulins known to be involved in specialized metabolite biosynthesis, and provides insights into a putative mode of cargo protein regulation via pore state modulation.