Table 1.

Characteristics and features of Class I and Class II HA synthases

Feature	Class I	Class II
Source organisms	Streptococci, amphibian, mammal, yeast, virus	Pasteurella multocida
Ability to be expressed as soluble active protein	No (must be associated with cell membrane)	Yes (amino acid residues 1–703)
Number of GT2 module	1	2
HA chain growth	At reducing end—Streptococcus, humans and mice At non-reducing end—Xenopus laevis and algal virus	At non-reducing end
Topology of the protein	Multiple membrane associated domains throughout the whole protein	Two catalytically independent domains, A1 and A2 attached to membrane via C-terminal region
Intrinsic polymerization	Processive	Non-processive
Size (amino acids)	417–588	972
Primer for initiation of HA synthesis?	No	Yes
Involvement of other molecules/proteins during HA translocation	Yes—involves lipid molecules (membrane bilayer)	Yes—may involve capsular polysaccharide transport machinery (more studies needed to confirm)