As a member of small heat shock proteins, HSP16.3 was identified as the major membrane-bound protein of Mycobacterium tuberculosis during stationary phase. Previous studies revealed that HSP16.3 was in a nonameric form in solution. Here, two-dimensional crystal of HSPI 6.3 molecules on lipid monolayer was obtained for the first time. The crystal exhibited p422 symmetry with lattice parameters a = b = 90 Angstrom, gamma = 90degrees. The projection map of untilted crystals showed that the basic unit of the crystal was a rod-like structure with two high-density regions. The three-dimensional map at 2.2 nm resolution revealed a rod-like structure with a dimension of 56Angstrom x 32Angstrom x 25Angstrom, similar to the dimeric forms of M. jannaschii HSP 16.5 and wheat HSP 16.9. Cross-linking experiments confirmed that HSP16.3 nonamers dissociated into dimers upon interaction with the positively charged lipid layer. Surface plasmon resonance measurements revealed that both electrostatic and hydrophobic forces involved in the formation of the 2D crystal on the lipid monolayer. These results provide a basis for further investigation on the unique dimeric structure of HSPI 6.3 and its functions in vivo. (C) 2003 Elsevier Inc. All rights reserved.