▁( AP C ) ▁protein ▁contains ▁ 1 1 ▁such ▁mot ifs ▁in ▁tan dem ▁arrangement ▁per ▁prot omer , ▁thus ▁capable ▁to ▁interact ▁with ▁several ▁ β - c aten in ▁mole cules ▁at ▁once . ▁ ▁Since ▁the ▁surface ▁of ▁the ▁AR M ▁domain ▁can ▁typically ▁accommod ate ▁only ▁one ▁pe pt ide ▁mot if ▁at ▁any ▁given ▁time , ▁all ▁these ▁prote ins ▁comp ete ▁for ▁the ▁same ▁cell ular ▁pool ▁of ▁ β - c aten in ▁mole cules . ▁This ▁competition ▁is ▁the ▁key ▁to ▁understand ▁how ▁the ▁W nt ▁signal ing ▁path way ▁works . ▁ ▁However , ▁this ▁" main " ▁binding ▁site ▁on ▁the ▁AR M ▁domain ▁ β - c aten in ▁is ▁by ▁no ▁means ▁the ▁only ▁one . ▁The ▁first ▁hel ices ▁of ▁the ▁AR M ▁domain ▁form ▁an ▁additional , ▁special ▁protein - prote in ▁interaction ▁pocket : ▁This ▁can ▁accommod ate ▁a ▁hel ix - form ing ▁linear ▁mot if ▁found ▁in ▁the ▁co activ ator ▁B CL 9 ▁( or ▁the ▁closely ▁related ▁B CL 9 L ) ▁– ▁an ▁important ▁protein ▁involved ▁in ▁W nt ▁signal ing . ▁Although ▁the ▁precise ▁details ▁are ▁much ▁less ▁clear , ▁it ▁appears ▁that ▁the ▁same ▁site ▁is ▁used ▁by ▁alpha - c aten in ▁when ▁beta - c aten in ▁is ▁local ized ▁to ▁the ▁ad her ens ▁j unction s . ▁Because ▁this ▁pocket ▁is ▁distinct ▁from ▁the ▁AR M ▁domain ' s ▁" main " ▁binding ▁site , ▁there