, ▁substr ate ▁specific ity ▁and ▁method ▁of ▁activation . ▁ ▁There ▁is ▁only ▁one ▁known ▁class ▁III ▁P I ▁ 3 - kin ase , ▁V ps 3 4 , ▁which ▁is ▁also ▁the ▁only ▁P I ▁ 3 - kin ase ▁expressed ▁in ▁all ▁e uk ary otic ▁cells . ▁In ▁humans ▁it ▁is ▁encoded ▁by ▁the ▁P IK 3 C 3 ▁gene . ▁In ▁human ▁cells ▁V ps 3 4 ▁associ ates ▁with ▁a ▁regul atory ▁sub unit , ▁p 1 5 0 . ▁ ▁Sub str ate ▁specific ity ▁V ps 3 4 ▁is ▁more ▁accur ately ▁described ▁as ▁a ▁ph osph at id y lin os it ol ▁ 3 - kin ase . ▁In ▁v ivo ▁V ps 3 4 ▁can ▁ph osph ory late ▁only ▁Ph osph at id y lin os it ol ▁to ▁form ▁ph osph at id y lin os it ol ▁( 3 )- ph osph ate ▁( P td In s ( 3 ) P ). ▁ ▁Function s ▁V ps 3 4 ▁was ▁first ▁identified ▁in ▁a ▁Sac char omy ces ▁c ere vis iae ▁( bud ding ▁ye ast ) ▁screen ▁for ▁prote ins ▁involved ▁ves icle - medi ated ▁vac u olar ▁protein ▁sorting ▁( h ence ▁V ps ). ▁A ▁number ▁of ▁prote ins ▁containing ▁a ▁ph osph o inos it ide ▁binding ▁domain ▁specific ▁for ▁P td In s ( 3 ) P ▁that ▁function ▁in ▁cell ular ▁protein ▁tra ff ick ing ▁have ▁been ▁identified