like ▁' head ' ▁domain ▁with ▁ATP ase ▁activity ▁( formed ▁by ▁the ▁interaction ▁of ▁the ▁N - ▁and ▁C - ▁terminal s ) ▁and ▁a ▁h inge ▁domain ▁that ▁allows ▁dim er ization ▁of ▁S MC s . ▁The ▁head ▁and ▁the ▁h inge ▁domains ▁are ▁connected ▁to ▁each ▁other ▁via ▁long ▁anti - parallel ▁co iled ▁co ils . ▁The ▁dim er ▁is ▁present ▁in ▁a ▁V - sh aped ▁form , ▁connected ▁by ▁the ▁h ing es . ▁ ▁The ▁N - term inal ▁domain ▁of ▁R AD 2 1 ▁contains ▁two ▁α - hel ices ▁which ▁forms ▁a ▁four ▁hel ix ▁bundle ▁with ▁the ▁co iled ▁co il ▁of ▁S MC 3 . ▁The ▁central ▁region ▁of ▁R AD 2 1 ▁is ▁thought ▁to ▁be ▁largely ▁un struct ured ▁but ▁contains ▁several ▁binding ▁sites ▁for ▁regul ators ▁of ▁coh es in . ▁This ▁includes ▁a ▁binding ▁site ▁for ▁SA 1 ▁or ▁SA 2 , ▁recognition ▁mot ifs ▁for ▁separ ase ▁cle av age ▁and ▁a ▁region ▁that ▁is ▁compet it ively ▁bound ▁by ▁P DS 5 A , ▁P DS 5 B ▁or ▁N IP BL . ▁The ▁C - term inal ▁domain ▁of ▁R AD 2 1 ▁forms ▁a ▁wing ed ▁hel ix ▁that ▁bind s ▁two ▁ β - sheets ▁in ▁the ▁Sm c 1 ▁head ▁domain . ▁ ▁Once ▁R AD 2 1 ▁bind s ▁the ▁S MC ▁prote ins , ▁S CC 3 ▁can ▁also ▁associate ▁with ▁R AD 2 1 . ▁When ▁R AD