Mistic: Cellular localization, solution behavior, polymerization, and fibril formation

Mistic represents a family of unique membrane‐associating proteins originally found in Bacillus subtilis (M110). As a fusion partner, it has been shown to assist overexpression of foreign integral membrane proteins in E. coli. We have expressed shorter Mistic homologs from other Bacillus species and surprisingly, unlike M110, found them abundant in the cytoplasm. These Mistic homologs including the corresponding shorter sequence (amino acids 27 through 110 of M110) exist as multimeric assemblies in solution in the absence of detergent. Crystals of Mistic from B. leicheniformis (M2) diffracted to 3.2 Å resolution, indicating that it exists as a multimer in the crystalline state as well. Moreover, we show that although M2 is mostly α‐helical, it tends to polymerize and form fibrils. Such oligomerization could potentially mask the charged surface of the monomeric Mistic to assist membrane integration.