The Gram negative bacillus Mannheimiahaemolytica is a natural inhabitant of the upper respiratory tract in ruminants and the most common secondary agent of the bovine respiratory disease complex. It is known to produce the extracellular neuraminidase NanH, which has a yet unknown biological role but is suspected to be important for bacterial adhesion to host cells, colonisation, capsule synthesis and biofilm formation. The structure of NanH is not known therefore, the functional domains of NanH, the tertiary structure and the residues involved in catalysis were predicted by sequence homology to the coordinates of other neuraminidases solved by crystallography. The catalytic domain was delimited from residues 23 to 435 and purified. The predicted catalytic residues were substituted in the recombinant NanH for confirmation of their role in hydrolysis of sialic acid. The function of the additional domains is unknown but analysis of NanH sequence and other associated genes found in the chromosome of M. haemolytica, suggest the presence of an autotransporter domain. The role of NanH in colonisation and infection is not known however, molecular characterisation is presented in this work. These data provide the basic knowledge required for future studies on using Nanh as a therapeutic and prophylactic target
