c aten in , ▁not ▁just ▁in ▁structure ▁but ▁also ▁in ▁function . ▁Thus ▁beta - c aten in ▁became ▁one ▁of ▁the ▁very ▁first ▁examples ▁of ▁moon light ing : ▁a ▁protein ▁performing ▁more ▁than ▁one ▁rad ically ▁different ▁cell ular ▁function . ▁ ▁Str ucture ▁ ▁Prote in ▁structure ▁▁ ▁The ▁core ▁of ▁beta - c aten in ▁consists ▁of ▁several ▁very ▁characteristic ▁repe ats , ▁each ▁approximately ▁ 4 0 ▁am ino ▁ac ids ▁long . ▁Term ed ▁arm ad illo ▁repe ats , ▁all ▁these ▁elements ▁fol d ▁together ▁into ▁a ▁single , ▁rig id ▁protein ▁domain ▁with ▁an ▁el ong ated ▁shape ▁– ▁called ▁arm ad illo ▁( AR M ) ▁domain . ▁An ▁average ▁arm ad illo ▁repeat ▁is ▁composed ▁of ▁three ▁alpha ▁hel ices . ▁The ▁first ▁repeat ▁of ▁ β - c aten in ▁( near ▁the ▁N - ter minus ) ▁is ▁slightly ▁different ▁from ▁the ▁others ▁– ▁as ▁it ▁has ▁an ▁el ong ated ▁hel ix ▁with ▁a ▁k ink , ▁formed ▁by ▁the ▁fusion ▁of ▁hel ices ▁ 1 ▁and ▁ 2 . ▁Due ▁to ▁the ▁complex ▁shape ▁of ▁individual ▁repe ats , ▁the ▁whole ▁AR M ▁domain ▁is ▁not ▁a ▁straight ▁rod : ▁it ▁poss esses ▁a ▁slight ▁curv ature , ▁so ▁that ▁an ▁outer ▁( con vex ) ▁and ▁an ▁inner ▁( con ca ve ) ▁surface ▁is ▁formed . ▁This ▁inner ▁surface ▁serves ▁as ▁a ▁lig and - binding ▁site ▁for ▁the ▁various ▁interaction ▁partners ▁of ▁the ▁AR M