▁The ▁SV 4 0 ▁large ▁T - ant igen ▁has ▁been ▁used ▁as ▁a ▁model ▁protein ▁to ▁study ▁nuclear ▁local ization ▁signals ▁( N LS s ). ▁It ▁is ▁imported ▁into ▁the ▁nucle us ▁by ▁its ▁interaction ▁with ▁import in ▁α . ▁The ▁N LS ▁sequence ▁is ▁PK K K R K V . ▁ ▁Inter action ▁with ▁p R b ▁via ▁the ▁L X C X E ▁mot if ▁ ▁SV 4 0 ▁large ▁T Ag , ▁other ▁poly om av irus ▁large ▁T ▁ant ig ens , ▁ad en ov irus ▁E 1 a ▁prote ins , ▁and ▁on c ogen ic ▁human ▁pap ill om av irus ▁E 7 ▁prote ins ▁share ▁a ▁struct ural ▁mot if ▁that ▁enc odes ▁a ▁high - aff inity ▁p R b - binding ▁domain . ▁ ▁This ▁mot if ▁is ▁character ized ▁by ▁an ▁Asp , ▁As n ▁or ▁Th r ▁resid ue ▁followed ▁by ▁three ▁invariant ▁am ino ▁ac ids , ▁inter sp ers ed ▁with ▁non - con ser ved ▁am ino ▁ac ids ▁( design ated ▁by ▁x , ▁where ▁x ▁cannot ▁be ▁a ▁L ys ▁or ▁Arg ▁resid ue ). ▁A ▁neg atively ▁charged ▁region ▁frequently ▁follows ▁car box y - term inal ▁to ▁the ▁p R b - binding ▁domain . ▁▁ ▁{ A sp / As n / T hr } ▁– ▁Le u ▁– ▁x ▁– ▁C ys ▁– ▁x ▁– ▁G lu ▁– ▁x ▁– ▁... ▁{ neg atively ▁charged ▁region } ▁ ▁H ydro