The major porin in this organism, forms a water-filled channel which favors the permeation of cations, amino acids, iron Fe(3+) and less efficiently phosphate. Does not transport Fe-ExoMS, the predominant siderophore. Plays a role in transport of beta-lactamase and hydrophilic fluoroquinolone antibiotics such as norfloxacin as well as chloramphenicol. There are about 2400 porins in wild-type, 800 in an mspA deletion and 150 in a double mspA-mspC deletion. Different conductance values with maxima at 2.3 and 4.6 nanosiemens might be caused by a simultaneous reconstitution of MspA channels into the membrane or by the existence of different MspA conformations. Forms very stable octamers. Isolated as a 100 kDa complex that can be reduced to monomers upon boiling in 80% dimethyl sulfoxide for 15 minutes. Structures show a goblet with the wide end on the exterior of the outer membrane and a central channel. It is not known if mixed oligomers of MspA with other Msp subunits form in vivo. Constitutively expressed. The rim domain (28-96 and 152-211) is accessible from the external milieu and is partially embedded in the outer membrane. The stem domain (131-148) is inaccessible from the outside and embedded in the outer membrane, while loop 6 (121-127) is thought to be in the periplasm. Single deletion has decreased permeability to the antibiotic cephaloridine (9-fold), glucose (4-fold), serine, phosphate and iron Fe(3+). Growth rates are wild-type, due to the other Msp-type porins. A double mspA-mspC deletion takes up less cephaloridine, glucose, serine and phosphate than the single mspA deletion and grows slower than wt or single mspA deletion. A triple mspA-mspC-mspD deletion grows even slower and has reduced Fe(3+) transport compared to wild-type. Belongs to the mycobacterial porin (TC 1.B.24) family.