Table 2 Characteristics of the Kell system

Kell protein

93 kDa type II membrane glycoprotein

Short N-terminal intracellular segment with one cysteine residue

Single transmembrane section

Large extracellular domain (665 amino acids with 15 cysteine residues)

One cysteine residue (Cys72) in extracellular domain binds XK protein

Homology to certain zinc endoproteases

Demonstrates endothelin-3 converting enzymatic activity

Encoded by the KEL gene (7q33)

K0 (null) RBC lack Kell antigens but have enhance XK activity

KEL gene is inherited autosomally

XK protein

Molecular weight of 50.9 kDa

10 transmembrane segments

Short N-terminal domain (intracellular)

Large C-terminal domain (intracellular)

A large hydrophilic loop (Figure 3)

Single cysteine residue that binds covalently to Cys72 on Kell

Loss of XK leads to McLeod syndrome that has X-linked inheritance