High-level heterologous expression and secretion in rapidly growing nonpathogenic mycobacteria of four major Mycobacterium tuberculosis extracellular proteins considered to be leading vaccine candidates and drug targets

Abstract

Mycobacterium tuberculosis, the primary etiologic agent of tuberculosis, is the world's leading cause of death from a single infectious agent, and new vaccines and drugs to combat it are urgently needed. The major extracellular proteins of M. tuberculosis, which are released into its phagosome in macrophages, its host cells in humans, are leading candidates for a vaccine and prime targets for new drugs. However, the development of these biologicals has been hampered by the unavailability of large quantities of recombinant extracellular proteins identical to their native counterparts. In this report, we describe the heterologous expression and secretion of four major M. tuberculosis extracellular proteins (the 30-, 32, 16-, and 23.5-kDa proteins--the first, second, third, and eighth most abundant, respectively) in rapidly growing, nonpathogenic mycobacterial species. Multiple attempts to obtain secretion of the proteins by using Escherichia coli- and Bacillus subtilis-based expression systems were unsuccessful, suggesting that high-level expression and secretion of these Mycobacterium-specific proteins require a mycobacterial host. All four recombinant proteins were stably expressed from the cloned genes' own promoters at yields that were 5- to 10-fold higher than those observed for the native proteins. The four proteins were purified to apparent homogeneity from culture filtrates by ammonium sulfate precipitation and ion-exchange and molecular sieve chromatography. The recombinant proteins were indistinguishable from their native counterparts by multiple criteria. First, N-terminal amino acid sequence determination demonstrated that processing of the leader peptides was highly accurate. Second, sodium dodecyl sulfate-polyacrylamide gel electrophoresis analysis revealed identical migration patterns. Third, mass spectrometry analysis confirmed that differences in mass were < or = 5 Da. A homolog of the M. tuberculosis 30-kDa protein was identified in M. smegmatis by means of DNA analyses and immunoscreening. This is the first time that secretion of recombinant M. tuberculosis extracellular proteins in their native form has been achieved. This study opens the door to mass production of correctly processed and secreted extracellular proteins of M. tuberculosis in a heterologous host and allows ready evaluation of their biologic and immunologic function.