Because sFlt-1 lacks the transmembrane domain that typically embeds tyrosine kinase receptors in the cell membrane, sFlt-1 travels freely in the blood circulation, and thus can travel from the tissue in which it is originally secreted to other areas of the body. As it contains the same extracellular domain as Flt-1, sFlt-1 competes with Flt-1 to bind VEGF and PIGF, effectively reducing serum concentrations of these two angiogenic growth factors. Though sFlt-1 can effectively dimerize, its lack of a kinase domain means that no tyrosine phosphorylation occurs upon ligand binding. As a result, sFlt-1 effectively sequesters agonists of Flt-1, and has been implicated as a regulator of this receptor in the kidney, liver, and brain.
