Group-1 biosynthesis

Schematic representation of Group-1 capsular polysaccharide biosynthesis and surface assembly

group-1 image

Group-1 Proteins

WbaP : Initial Glycosyl transferase

UDP-Gal : Undecaprenol Gal-1-phosphate

WbaZ : Glycosyl transferase

WbaO : Glycosyl transferase

WbaN : Glycosyl transferase

Wzx : Repeat unit exporter ( Flippase)

Wzy : Repeat unit polymerase

Wza : Putative outer membrane lectin (Translocon)

Wzb : Putative acid phosphatase

Wzc : Putative trans membrane protein (Autokinase)

Wzi : Role in final stages in Capsule assembly

  1. Assembly via the Wzy-dependent pathway begins in the cytoplasm with the synthesis of lipid-linked polysaccharide repeat units
  2. The first sugar galactose-1-P from UDP galactose is added by an initiating glycosyl transferase (WbaP) to the lipid carrier molecule undecaprenyl phosphate (UndP) in the form of an activated nucleotide precursor at the inner leaflet of the inner membrane (IM), resulting in a pyrophosphate linkage with the carrier (undecaprenyl pyrophosphate, UndPP)
  3. Undecaprenol has also been implicated as a scaffold for organizing proteins including GTs, and the lipid: protein complexes may alter the biophysical properties of the local membrane environment, perhaps aiding the flipping activity
  4. The export process is a complex one. Subsequent sugar additions by specific glycosyltransferases result in the synthesis of an individual UndPP linked polysaccharide repeat unit (referred to as an ‘K’ unit), after which a series of integral IM assembly proteins take over
  5. The UndPP-linked repeat units are translocated from the inner to the outer leaflet of the IM by the flippase Wzx, (Wzx proteins have been identified in Gram-negative and Gram-positive bacteria, as well as in archaea, and are classified as part of the polysaccharide transporter (PST) family of proteins.) where they are polymerized by Wzy
  6. Sugar-unit addition occurs at the reducing terminus of the growing chain, the length of which is regulated by the polysaccharide co-polymerase Wzz, resulting in organism-specific preferred modal lengths in Gam positive organisms. This polymerized glycan is then anchored to lipid A-core oligosaccharide by WaaL in the case of O-Ag to form a mature LPS molecule
  7. Incase of these organisms, Wzc the inner (cytoplasmic) membrane is a tyrosine autokinase. The C-terminus must be cytoplasmic to facilitate tyrosine phosphorylation like Wzz
  8. The Wzc-Wzb proteins are highly conserved in group 1 capsules and colanic acid biosynthesis systems. Wzb are both essential for group 1 and 4 capsule assembly, suggesting that Wzc must cycle its phosphorylated state
  9. Wza is an outer membrane protein involved in translocation of the capsular polysaccharide. It forms a channel and interacts with Wzc periplasmic domain
  10. Wzi is a conserved gene incase of only group-1 capsular organisms. This is found to be absent in all other groups. Cps transcriptional promoter is located upstream of wzi. The role of this is to help in attachment of the capsular polysaccharide to the cell surface (Surface association). It is a heat modifiable beta barrel protein which is not essential for capsular biosynthesis.