▁as ▁substr ates . ▁This ▁protein ▁also ▁trans ports ▁gl uc ur on ides ▁and ▁sul f ate ▁conjug ates ▁of ▁ster oid ▁h orm ones ▁and ▁b ile ▁sal ts . ▁Altern ative ▁sp lic ing ▁by ▁ex on ▁delet ion ▁results ▁in ▁several ▁sp lice ▁variants ▁but ▁maintain s ▁the ▁original ▁open ▁reading ▁frame ▁in ▁all ▁forms . ▁ ▁Str ucture ▁ ▁AB CC 1 ▁is ▁a ▁ 1 9 0 ▁k Da ▁protein ▁that ▁contains ▁two ▁memb rane - sp anning ▁domains ▁of ▁hydro ph ob ic ▁nature ▁and ▁two ▁nucle ot ide ▁binding ▁domains . ▁Each ▁memb rane - sp anning ▁domain ▁is ▁made ▁up ▁of ▁six ▁α - hel ices . ▁In ▁addition , ▁the ▁protein ▁also ▁contains ▁a ▁third ▁memb rane - sp anning ▁domain ▁that ▁sets ▁it ▁apart ▁from ▁other ▁trans por ters ▁within ▁the ▁ATP - binding ▁c ass ette ▁family ▁of ▁trans por ters . ▁The ▁two ▁nucle ot ide ▁binding ▁domains ▁have ▁a ▁functional ▁asym met ry ▁that ▁plays ▁a ▁significant ▁role ▁in ▁the ▁ability ▁of ▁ATP ▁to ▁power ▁the ▁trans porter . ▁The ▁first ▁nucle ot ide ▁binding ▁domain , ▁which ▁is ▁deleg ated ▁N BD 1 , ▁is ▁responsible ▁for ▁the ▁strong ▁att raction ▁of ▁ATP ▁to ▁the ▁trans porter . ▁The ▁second ▁nucle ot ide ▁binding ▁domain , ▁N BD 2 , ▁is ▁the ▁domain ▁responsible ▁for ▁the ▁hydro ly s is ▁of ▁ATP . ▁This ▁asym met ry ▁is ▁specific ▁to ▁the ▁C ▁sub family ▁of ▁ABC ▁trans por