uc ine - Rich ▁Cit ron ▁Hom ology ▁Domain ▁( CN H ) ▁ ▁Inter domain ▁- ▁Connect s ▁the ▁kin ase ▁and ▁C N H ▁domains , ▁facil it ates ▁protein - prote in ▁interactions . ▁Although ▁it ▁has ▁been ▁identified , ▁its ▁struct ural ▁components ▁and ▁functionality ▁are ▁currently ▁poor ly ▁understood ▁ ▁Altern ative ▁sp lic ing ▁of ▁the ▁M AP 4 K 4 ▁gene ▁yields ▁five ▁functional ▁is o forms , ▁all ▁of ▁which ▁display ▁fully ▁hom olog ous ▁kin ase ▁and ▁C N H ▁domains ▁but ▁differences ▁in ▁the ▁inter domain ▁domain . ▁While ▁the ▁bi ological ▁significance ▁of ▁these ▁is o forms ▁remains ▁to ▁be ▁determined , ▁it ▁can ▁be ▁spec ulated ▁that ▁such ▁variations ▁alter ▁and ▁determine ▁M AP 4 K 4 ' s ▁interactions ▁with ▁other ▁prote ins ▁and ▁factors , ▁ultimately ▁leading ▁to ▁the ▁activation / in hib ition ▁of ▁different ▁bio chem ical ▁and ▁phys i ological ▁cas c ades . ▁ ▁The ▁m amm al ian ▁class ▁of ▁Ste 2 0 ▁kin ases ▁require ▁ph osph ory lation ▁at ▁specific ▁sites ▁for ▁full ▁activity . ▁Primary ▁ph osph ory lation ▁at ▁the ▁activation ▁site ▁in ▁their ▁kin ase ▁domain ▁is ▁believed ▁to ▁cause ▁a ▁conform ational ▁change ▁in ▁the ▁protein , ▁stabil izing ▁the ▁structure ▁of ▁its ▁activation ▁segment ▁to ▁allow ▁suitable ▁substr ate ▁binding . ▁Second ary ▁sites ▁also ▁require ▁ph osph ory lation ▁for ▁the ▁en zym e ▁to ▁assume ▁full ▁activation ▁and ▁is ▁achieved ▁via ▁aut oph osph ory