Cloning of the mspA gene encoding a porin from Mycobacterium smegmatis

Porins form channels in the mycolic acid layer of mycobacteria and thereby control access of hydrophilic molecules to the cell, We purified a 100 kDa protein from Mycobacterium smegmatis and demonstrated its channel-forming activity by reconstitution in planar lipid bilayers. The mspA gene encodes a mature protein of 184 amino acids and an N-terminal signal sequence, MALDI mass spectrometry of the purified porin revealed a mass of 19 406 Da, in agreement with the predicted mass of mature MspA. Dissociation of the porin by boiling in 80% dimethyl sulphoxide yielded the MspA monomer, which did not farm channels any more, Escherichia coli cells expressing the mspA gene produced the MspA monomer and a 100 kDa protein, which had the same channel-forming activity as whole-cell extracts of M. smegmatis with organic solvents. These proteins were specifically detected by a polyclonal antiserum that was raised to purified MspA of M. smegmatis, These results demonstrate that the mspA gene encodes a protein of M. smegmatis, which assembles to an extremely stable oligomer with high channel-forming activity, Database searches did not reveal significant similarities to any other known protein. Southern blots showed that the chromosomes of fast-growing mycobacterial species contain homologous sequences to mspA, whereas no hybridization could be detected with DNA from slow growing mycobacteria, These results suggest that MspA is the prototype of a new class of channel-forming proteins.