Structural characterization of the type II secretion system of Aeromonas hydrophila

Abstract

The exeC gene, found in the gram-negative bacteria Aeromonas hydrophila codes for a 31 kDa, three domain, bitopic inner membrane protein. The components of the ExeC protein include an amino-terminal cytoplasmic domain, a trans-membrane helix and two periplasmic domains. The two periplasmic domains are involved in recognition and selection of protein substrates which are subsequently transported across the outer membrane and free of the cell. This study focuses exclusively on the two periplasmic domains referred to hereafter as the HR and the PDZ domains. Three constructs were used throughout the course of this study. Two of them were designed, cloned and expressed for this study. The third is a result of previous work. Two constructs contained both the HR and PDZ domains while the other consists of the amino-terminal periplasmic HR domain. Only one construct was used to grow single crystals for analysis by X-ray crystallography. Crystals comprised of the PDZ domain from a degraded construct grew in a hexagonal space group with a hexagonal bi-pyramidal morphology. Crystals diffracted anisotropically to a maximum resolutions of 2 Å along the c axis and 3 Å in the a/b plane. Anisotropy in combination with twinning drastically complicated structure solution. Efforts toward elucidating the crystal structure will be discussed.