1.S.7.  The Bacterial/Archaeal Nanocompartment Encapsulin Shell Protein2 (BANC-SP2) Family

These proteins may be distantly homologous or similar in size to those in TC family 1.S.6, They are all called 'encapsulins'.  Pores in encapsulins have been engineered as a general strategy to improve protein-based enzyme nanoreactor performance (Kwon et al. 2024). Cargo enzymes include ferroxidases involved in iron storage, peroxidases involved in oxidative stress resistance, cysteine desulfurases involved in sulfur storage, and terpene cyclases involved in terpenoid biosynthesis (Dutcher et al. 2024). Enzyme encapsulation inside encapsulin shells can convey a number of distinct advantages, such as the formation of an optimized reaction environment, enzyme co-localization, intermediate sequestration, and the exclusion of unwanted proteins and molecules. The encapsulin shell may also serve to regulate cargo enzymes by modulating the flux of small molecule metabolites through its pores whilst stabilizing the oligomeric state of enzymatic cargo. Encapsulins are found in at least 35 prokaryotic phyla and have been classified into four distinct families based on operon structure and shell protein phylogeny (6, 33). Family 2, the most abundant class of encapsulins, is further subdivided into Family 2A and Family 2B based on the absence or presence, respectively, of a putative cyclic nucleotide binding domain (CBD) inserted into the E-loop of the HK97 fold encapsulin shell protein (Dutcher et al. 2024).