Homotrimer. When expressed in mutant CS384. Encoded by the cryptic lambdoid prophage DLP12. Belongs to the Gram-negative porin family. Could be the product of a pseudogene, as in E.coli K12 the nmpC open reading frame is interrupted by an IS5 insertion and generates a hybrid open reading frame that is not expressed. In strains which have undergone selection for suppressors of ompB mutations (for example, mutant strain CS384) nmpC is expressed.