STRUCTURE OF AN IMMUNODOMINANT 38-kDa PROTEIN ANTIGEN b (Pab) FROM MYCOBACTERIUM TUBERCULOSIS. Nand K. Vyas, Meenakshi N. Vyas, Abha Choudhary, Zengyi Chang and Florante A. Quiocho, Department of Biochemistry and Howard Hughes Medical Institute. Baylor College of Medicine. Houston, TX-77030.

Mycobacterium tuberculosis, an etiological agent of tuberculosis, infects about one-third of the world's human population. Tuberculosis remains the largest cause of deaths (3 million each year) from a single infectious agent. Among well characterized secreted protein antigens, the 38-kDa protein antigen b (Pab) from M. tuberculosis is of great current interest in the immunology of tuberculosis because its B and T cell epitopes are species-specific and immunodominant. Amino acids sequence of the 38-kDa protein has 30 % similarity with the periplasmic phosphahe-binding protein (PBP) from Escherichia coli (E. coli). The 38-kDa gene from M. tuberculois has been subcloned and overexpressed in the nonpathogenic E. coli for structure-function studies. We have determined an X-ray structure of the recombinant 38-kDa at 3.0 Å resolution by the MIR method. Results of the 38-kDa structure determination and further refinement will be presented. The structure of the 38-kDa will be used for topographic mapping of known B and T cell epitopes to understand cooperation between B and T cells in immune responses. In addition, the 38-kDa structure will be used for comparison with the known structure of the PBP (E. coli). Crystallization of the 38- kDa by vapor diffusion and repeated seeding methods produced crystals in two distinct forms. The orthorhombic form, space group P2_l2_l2, has cell dimensions: a= 125.45 Å b= 72.27 Å and c= 73.43 Å, whereas monoclinic form, space group P2₁, has cell dimensions: a= 67.42 Å b= 113.38 Å, c= 42.68 Å and ß= 108.53deg.. Asymmetric unit of each crystal form contains two molecules of the 38-kDa. Both crystal forms diffract X-rays to 2.0 Å resolution.