This work was supported by the South African Medical Research
Council (Award to the MRC/UCT Receptor Biology
Research Unit); the Claude Leon Foundation, South Africa (Postdoctoral
Research Fellowship: C.L.N.); the University of Cape
Town (URC Fellowship: R.C.A.); The University of Pretoria
(RDP grants: C.L.N. and R.C.A.); and the National Research
Foundation, South Africa (CSUR Grant : C.L.N.).