The type b polysaccharide capsule of Haemophilus influenzae type b is a polymer of ribose and ribitol-5-phosphate and is a critical determinant of virulence. Expression of the type b capsule is dependent upon the cap b locus, which consists of three functionally distinct regions, designated regions 1 to 3. Region 3 contains the hcsA and hcsB genes, which share significant homology with genes that have been implicated in encapsulation in other pathogenic bacteria. Inactivation of hcsA alone resulted in accumulation of polysaccharide in the periplasm and a partial decrease in surface-associated polysaccharide. Inactivation of hcsB alone or of both hcsA and hcsB together resulted in accumulation of polysaccharide in the periplasm and complete loss of surface-associated polysaccharide. The hcsA and hcsB gene products apparently have complementary functions involved in the transport of polysaccharide across the outer membrane (Sukupolvi-Petty et al, 2006).

HcsB (422aas) exhibits an unusual hydropathy profile with 3 hydrophobic peaks centered at residues 40, 180, and 350. An NCBI BLAST search revealed homology with KpsS of E. coli (capsular polysaccharide synthesis as well as proteins annotated as putative glycosyltransferase and as capsular polysaccharide exporter (e.g. KpsC)). The evidence presented by Sukupolvi-Petty et al. (2006) suggests that it is an outer membrane transporter, but its mechanisms of action and energization are not known.

The reaction believed to be catalyzed by members of this family is:

Capsular polysaccharide (out) → Capsular polysaccharide (cell surface)