1.B.79 The Porin-Sphingomyelinase Fusion Protein, SpmT (SpmT) Family
The Mycobacterium tuberculosis (Mtb) protein, Rv0888, called SpmT (sphingomyelinase of Mycobacterium tuberculosis), possesses potent sphingomyelinase activity, cleaving sphingomyelin, a major lipid in eukaryotic cells, into ceramide and phosphocholine, which are then utilized by Mtb as carbon, nitrogen and phosphorus sources. An spmT deletion mutant could not grow on sphingomyelin as a sole carbon source and replicated poorly in macrophages, indicating that Mtb utilizes sphingomyelin during infection. SpmT is an unusual membrane protein with a surface-exposed C-terminal sphingomyelinase domain and a putative 8 β-stranded N-terminal barrel-forming channel domain that mediated glucose and phosphocholine uptake across the outer membrane in an M. smegmatis porin mutant. Erythrocyte membranes contain up to 27% sphingomyelin. The finding that SpmT accounts for half of Mtb's hemolytic activity is consistent with its sphingomyelinase activity and the observation that SpmT levels are increased in the presence of erythrocytes and sphingomyelin by 5- and 100-fold, respectively (Speer et al. 2015).