Complex sentence: Tie-1 and Tie-2 comprise the cell-surface receptors which bind and are activated by the angiopoietins, (Ang1, Ang2, Ang3, Ang4) which are protein growth factors required for the formation of blood vessels (angiogenesis).
Split into simpler sentences: Tie-1 and Tie-2 comprise the cell-surface receptors which bind and are activated by the angiopoietins, (Ang1, Ang2, Ang3, Ang4). The angiopoietins are protein growth factors required for the formation of blood vessels (angiogenesis).